That’s because it’s not an economic policy but a humanistic one. Stable housing should be a right. If rent control was the default, then obviously no renter would vote for arbitrary rent increases in exchange for maybe, someday in the future, rent going down due to increased housing supply.
You can feel however you want about it, but people will vote for rent control if they start getting squeezed out of their housing. Hate it as an economic policy? Then make sure enough housing gets built before people get squeezed.
It doesn’t really harm people. Housing crisis cities with rent controls are built out to the limits of their zoning. This suggests zoning is the limiting factor and not rent control.
Call it meaningless if you want, but this is a policy that renters getting priced out of their housing markets will eagerly vote for. And prioritizing existing residents over newcomers feels just to me. Focus on making it easier to build new housing instead of getting pissy about the safety net.
"Newcomers" to the rental market include every person who already lives in that city and hasn't moved out of their parents' house yet. Why should they be penalized even more for not being born sooner?