Building transit does not guarantee you get density. There are regulatory hurdles at play. In California, for example, cities will often be forced to up-zone the immediate couple of blocks around a station, but then everything else beyond that remains zoned for single-family housing, or at least zoned for far less density than the market could support.

Moreover, there's a time gap between when housing is built and when it becomes economically accessible lower down the income ladder. New housing is expensive, even absent regulatory hurdles, and affordability mandates can't magically change financial viability. New housing can have immediate effects on housing costs, but indirectly on the costs of older housing; and on a regional scale it takes awhile to incentivize people at the periphery to move inward.

Long-story short, to get from the current equilibrium to a more optimal equilibrium requires careful, coordinated policy management across decades. We could and should start, but we live in an age where if people don't see results in 6 weeks, they complain; and even when there are results, they still complain. They complain, vote people out, and vote in new people that promise more immediate results, but invariably can't deliver meaningful results.[1] I mean, this is how democracy has always worked, but the public reactions are far more compressed, while the necessary time spans for effective change are no shorter than they were before.

[1] Case in point: rather than improving public transit, the popular panacea du jure is free fares. Free fares will not improve public transit in any way shape or form; if anything, they'll hasten its degradation. They'll make it cheaper for some, but for the most part not those at the bottom of the ladder, who already enjoy free transit. These gimmicks are popular because they're immediate.