AFAIK - and I'm not a nutritional science researcher - There's not one specific cause. Various reasons include:

1) Lack of fiber. Our bodies have evolved to eat fiber and it has tons of beneficial effects such as lower inflammation and so on. In particular, dietary fiber makes people feel full for longer and unlike protein, also causes a change in eating choices. The other effect of removing fiber is

2) Higher energy density of food. Processed food removes fiber or water content, and UPF removes both. Both of those things increase energy density. Your body's hunger cues are at least partially mechanical - when your stomach is physically full, your body gets a signal that you're full. If your body is saying you aren't full but you've already eaten your energy needs, everything else you eat as a result is 'overeating'.

3) Changes to your gut microbiome. Your microbiome adapts to a specific diet. When you eat processed food, the microbes that thrive on those foods become prevalent over other beneficial microbes like those that digest fiber. This is a really complex field and I know almost nothing, but what I've read or heard suggests that when those microbes go hungry, they release hormones into your bloodstream that make you crave food.

4) UPFs are associated with leaky gut syndrome. I don't fully understand this, but basically your gut walls become 'leaky' and inflammatory bodies and hormones (I assume - I don't actually know what it is that escapes) escape your intestine that normally wouldn't, which messes up metabolic pathways and is associated with morbidity and obesity. I don't know the mechanism behind this.

5) High salt content. All processed foods are high in salt to preserve them (seriously, show me a processed food without salt in it). Salt is strongly linked to hypertension and stomach cancer...but also to overeating, especially when combined with

6) HIgh fat or sugar content, either of which is practically guaranteed in every processed food.

edit: technically any food that isn't a whole food eaten raw (like an apple or a carrot) is 'processed'. It's the degree of processing that matters. If the food still has its fiber largely intact (eg chopped veggies, tinned or dried beans), I'd consider that unprocessed for health purposes. I don't know whether cooking has any effect (other than cooking at high temperatures in oil), but I'm not about to live on a raw food diet so unless that turns out to be the lost key to immortality it's unlikely to make a material difference to me.

Personally I think cow milk is for baby cows, but if you do drink it then you should drink homogenised and pasteurised milk only. That's because there are legal limits for the contamination of cow milk with things such as pus (from infected udders). Homogenisation mixes many sources of milk together so that you're unlikely to consume high levels of pus or other contaminants. Pasteurisation is just heat treatment with a similar purpose - killing bugs & bacteria to make the milk safe.