(a) It's not clear why YC could run an apartment complex better or cheaper than the many companies who already do it (including some YC-funded startups like https://zeusliving.com)
(b) Founders tend to be independent-minded people who like to do things their own way.
why would existing companies be offering cheaper? The price will be set by the market, and any saving will be taken as profit, not passed on to the tenants.
The easiest way to insure against rent increase is buy a big chunk of local land. Actually offering it to your own startups just means they reduce overheads a little, provide an extra perk, and a small competitive advantage.
What is the opportunity cost of that land investment? Is YC really better off getting into the landlord business rather than using the same money to fund more companies?
(b) Founders tend to be independent-minded people who like to do things their own way.