Rents are outpacing pay and pay increases. Stiff increases (like the 20% I got, 50% my neighbors received) displace long term residents, including families. I am in the LA area and our school district lost 3 students as a result of my neighbors' rent increases, and other long-time neighbors moved.
The short answer is this. An effective community is a complex and wonderful thing for a neighborhood to achieve. Letting "the market" determine who lives in your neighborhood is a weird way for human beings to construct communities. Rich people can buy into any neighborhood, evict poor people and reconstruct what the neighborhood is around their own notions of community forcing the evicted to forage wherever they can find what they can afford. Bringing differently talented people together in communities with rich, different cultural backgrounds, career aspirations, etc is a skill in no way reducible to "letting the market" dictate gentrification. And if a democratic way to construct communities and neighborhoods means anything, it means NOT allowing rent inflation to dictate who lives somewhere rather than democratic decision-making. The bottom line: creative, democratically organized and run communities are fundamentally incompatible with capitalism and market mechanisms.