It's because there is a conflict of desires that exist within cities. People want cities with thriving economies, jobs with rising wages, world class amenities

It's because there is a conflict of desires that exist within cities. People want cities with thriving economies, jobs with rising wages, world class amenities and institutions. However, they also want the amenities that are only available in mid sized cities. The ability to have a single family home a 20 minute drive from downtown with a yard with available parking, undeveloped nature and farm land that isn't over an hour away, parks that aren't always packed, and many other things that are incredibly hard to add back into the city once it starts to be developed in any serious way. The YIMBY solution is to let the market decide, which will always demand increased transformation of cities with good economies, when we'd build a better world for everyone should we put in more effort to transform smaller cities into the places with jobs, good wages, with high quality institutions and amenities. Tons of tech workers move to the west coast not because they've always wanted to, but because they know that they're restricting their career opportunities should they not move.