Modern homes have much better insulation and energy efficiency (like LED bulbs), etc. But I think it's also true that a lot of builders can get away with cutting corners, and the materials that go into homebuilding are also often of lesser quality (like wallboard from China that outgasses toxic sulfur fumes). Modern plumbing is mostly snap-fit plastic tubing rather than copper pipes. It's a LOT cheaper to install. Supposedly lasts as long or longer, but it's still shitty plastic.

Shitty wildcat builders throwing down entire subdivisions of deeply defective homes is a serious problem in the Phoenix area, I know that for a fact.