It's all NIMBYism. We absolutely could shit out a standardized reactor design and build as many as we need but you can't get people to agree that we should do that, and even a lot of the people who DO want nuclear power want it as far away from them as possible.

Too many decades of mis/disinformation around things like TMI and Chernobyl have ruined several generations of people's opinions on being near nuclear even if they generally approve of it. (And by near, I mean in the same state as them, even.)

This is strictly a public opinion problem, and one reason solar and wind is expanding so rapidly is nobody has any major objections to those.