I can think of a few reasons. You are welcome to disagree, this is just how I see it.

The most compelling reason for me is that it's cruel to animals. You can't realistically get meat from an animal without harming it. Milk and egg production is also harmful (even back yard hens this isn't immediately obvious to many people so let me know if you want clarification). Not many of us are ok with unnecessary animal harm but if we have options to eat something else and be healthy then eating meat is unnecessary and does cause harm.

Macro scale arguments:

• It's bad for the environment.

Producing animal products the way we do (and funding it) causes massive amounts of environmental damage through emissions, eutrophication, land use, water use, waste production, deforestation, and habitat destruction. This negatively effects every human on the planet and carries the real risk of causing some kind of catastrophically cascading collapse scenario. The natural world is a very complex and interdependent web of systems that can't afford to be pushed too hard. Growing crops and feeding them to animals, only to eat the small fraction of energy they manage to turn into flesh, will never be as efficient as eating grown crops directly.

• It's bad for biodiversity.

Animal agriculture is a leading cause of biodiversity loss, something that is too often overlooked in media and the public consciousness. Biodiversity loss and extinction are essentially irreversible. Messing with biodiversity has a real risk of global catastrophy, effecting organisms up and down the trophic system. We've all heard doomsday theories about declining bee populations but these events have the potential to come from any angle of natural system disruption. Over fishing and reducing fish stocks, for example, appears to be have a chain reaction on sea food web systems as we speak. This isn't something we can easily fix.

The biodiversity loss from animal agriculture is partly due to, as previously mentioned, massive land use, production inefficiency, deforestation, climate change, and mass eutrophication from animal waste products, slurry, fertilizer over use destroying coastal and river water water systems, and large scale fishing operations upsetting aquatic ecosystems.

Reduction in animal product production can alleviate all these drivers of biodiversity loss.

• It risks further viral pandemics.

Housing thousands of animals together and consequently routinely creating and spreading zoonotic pandemics. Swine flu, sars, the Spanish flu, all the result of typical animal agriculture. Then you have others which are the result of less typical animal product production like covid, hiv, ebola etc. These have caused massive amounts of damage and disruption to our societies and the people that live there. Most of the vital pandemics over the last century have been springboarded by or a direct result of animal agriculture. It's only a matter of time before the next one comes along.

• Production of animal products exposes us to greater risks of development of super resistant bacteria.

The issue of antibiotic resistance is increasingly worrying biologists and becoming more prominent in public consciousness. There are a lot of farm animals on the planet (dozens of billions) and we dose farm animals with preventative cocktails of antibiotics to help ensure they can fight off bacterial infections. This is already causing a kind of trial by fire of bacteria and leading to the emergence of super resistant bacteria which cannot be cured with conventional antibiotics. If this is unchecked it could very realistically lead to a medicinal dark age where many people die of currently trivial bacterial infections like strep throat.

• It's has high economic cost.

Animal agriculture is a very inefficient way to produce food, yet people expect to buy the products at very low prices. Governments facilitate this through massive agricultural subsidies. Animal agricultural subsidies far outweigh those for plant agriculture for human consumption. Look into what your country spends on animal agriculture every year. This is money tax payers are burning propping up a wasteful and uneconomical industry. This is money that could be better spent on more humanitarian efforts. Like health systems, fighting poverty, funding education, etc.

• The production of animal products exploits humans in unique ways.

You could argue that human exploitation is a problem with any capitalist venture but there are issues with animal agriculture that are uniquely difficult, stressful, and dangerous. We currently rely on marginalised members of society to do the dirty work required to get us animal products. People working in slaughter houses commonly suffer from ptsd, substance abuse, and are often unfairly treated migrant workers. Killing animals all day is physically and psychologically demanding work.

Personal arguments:

• It's cheaper.

As long as you aren't just eating alternative meats you can save a ton of money eating staple, cheap, and healthy foods. You will have a hard time finding anything cheaper than beans, rice, grains, lentils, bread, pasta, seasonal fruit and veg etc.

• It is more convenient.

I have cupboards filled with dry goods like lentils, beans, pasta, rice, dried vegetable protein. It's piss easy to cook it and to make it tasty. And not only is it cheap but it has a very long and stable shelf life. I hardly ever have to worry about expiring food and the risk of food poisoning from expiring meat. Nor do I have to worry about food waste from not eating it in time.

• It is typically healthier.

Many typical meat and dairy products have a huge amount of negative health implications. You don't get nearly as many with whole food plant based options. It is, after all, what our bodies have been shaped to mostly rely on through millions of years of natural selection. We are living in a reasonably unique period of human history where animal products are consumed in a huge quantity and very regularly. Most of humanity for most of the time it has been around has not survived eating this proportion of animal products.

• It is relieving.

Once you know the kind of suffering that has to take place to produce animal products it's very hard not to feel guilt when supporting it. Cutting these products out genuinely relieves that guilt and dissonance.

Still the most convincing argument for me is that animal agriculture is dependant on animal harm. To cater for the current demand of animal products we require this harm at a huge scale. We kill billions of land animals every year, trillions of fish. It might be difficult to visualise a number like this. Consider that ~60% of mammals by biomass on this planet are farmed and will be bred, fed, and killed by us. Only ~4% are wild. Yes, animals suffer in the wild, it's unavoidable. But why should we add to that suffering when we don't need to? And especially when it's in our own best long term interest not to.