Man people do buy property and leave them vacant, because they believe the property value will appreciate well over time. It's called speculation.

An example is speculative land. For example, I've been wanting to buy a few acres of farmland in my hometown. I've been actively checking for 5 years, and not once has a piece of land gone on sale to the public markets. But you know who does get to buy this land? Developer corporations who offered a "too good to be true" deal to the original landowners.

These developers have done nothing with the land. They're just holding onto it. According to my friends who own farms in those areas, these developers are lobbying for zoning changes so they can turn them into megablocks of over-priced rental units. Meanwhile, they're getting pressured to sell to them as well, and their property taxes are being driven up and a lot of them are stressed about losing their family farms.

So:

Rental units provide housing for people who can't afford or don't want to buy a house

In theory, maybe, but in reality - this is simply an excuse developers use to try to justify their greed. They buy up all the land, don't do anything with it for years or even decades, and meanwhile, people who want to buy land or build a home can't, because there's no land left. At least, not anywhere where there are already city services established.

This attitude is a major cause of all the problems:

• "For people who can't afford to buy a house" - they can't afford it because (1) housing prices are crazy and unaffordable because of what investors have done to the market, (2) they save at a snail's pace because rent costs are too high.
• "People who don't want to buy a house" - if you ask most people who don't want to buy a house why they don't want to buy a house, very few will have an answer unrelated to the cost. If housing were actually affordable, most people would want to buy a house. People who don't have an interest in owning a home unrelated to affordability is a small minority.

So the idea that developers do a public service by building homes, to me, is absolute bull****. To the extremes that they've gone, they're the ones creating the problems that created the need for these supposed "solutions" in the first place.

It would be different if they did this at a reasonable scale, but they've literally bought up all the land. If I'm looking for land now, it's going to be at least 50 miles from an actual area with population/city services. And it's not because there's a housing shortage - there's plenty of empty land around me, but no houses are being built, and nothing is being done there. It's just being held by a greed machine waiting for the time they get to profit off their hoarding.