Tl;dr because new skyscrapers are more narrow than old ones, and narrower skyscrapers take more time to build than wide floor plans with comparable total floor space.

I wonder whether it's true for normal housing too, or reasons are quite different

Can we please start building affordable housing on artificial islands?

Buying one building and turning it into affordable housing isn't enough. We have to start thinking big. Only way to do that is expand into the Hudson river and outside of queens, Brooklyn and past long island as well. We need to start also building artificial islands out a couple miles too for future use and to stop hurricane waves.

This will also cut down on hurricane waves and flooding

There's no more space in NYC so this is what we need to do.

Makes sense, that now that we have unions and safety regulations, projects take longer to complete.