I don’t know why I have to explain this, but I’m not aware of any situation in which a renter builds equity in the underlying asset either.

Under rent control the situation goes from being extremely advantageous to the landlord to just being regular advantageous.

>enjoys the upsides, but can run away when there are downsides without any consequences?

Isn't that why landlords argue they should make profit, because they carry the risk?

Which downsides can they run away from? They pay for maintenance, property taxes, and so on. And still pay a premium over that.

They don't pay for these things if their rent is fixed.

The vast majority of rent controlled cities have rent control boards where landlords can request and increase in rent in case of exceptional circumstances. In those cases, the landlord must show that the rent is not enough to cover maintenance, and the increase is granted.

In general though, maintenance on average only costs ~150-300$ a month, so it's not necessary.