Because you don't have to buy an apartment. You don't have to pay anything close to the full value of the apartment up front to live in it. You pay something akin to a deposit, and you are sure to get it back with appreciated value if you move out. You don't have the hassle of buying and then selling associated with it. If you leave the co-op you get the portion of the co-op you own paid out and that's it.

The important distinction is that the apartments are paid for by the rent other tenants have paid, that accumulated and is then used for construction. That means the buy-in of a new member is not tied to the market value of the apartment at all, but rather what the co-op values it membership at.

The co-op has an incentive to increase the membership fees because doing so increases the value of all memberships. So if someone decides to leave the co-op they get more than what they paid in, making it a pretty good investment. Theoretically it's capped at demand for memberships, but afaik this has never been an issue.

Furthermore the co-op can offer these apartments however they wish, they aren't bound to a profit motive. They have the option to be more lenient or offer financial assistant to those who can't afford the buy-in. And growing the co-op is usually desirable because it makes a lot of services cheaper like not having to contract out gardening work but employing the gardeners itself.

I hope that makes sense, my family needs me.