Philly only has a less than ~5% vacancy rate on apartments. That's so low that the small amount of incoming supply isn't enough to have any meaningful impact on decreasing rates.

The answer to that is that we need to build even more of everything to account for all the demand. The clowns saying we shouldn't build new housing are the same people who bitch about how expensive rent is. They're too dumb to connect the lack for supply with the incoming population, and the subsequent increase in rent.