A few things. first off, we have code now. Until the 1950s you could do whatever you wanted. But nowadays there a bunch of things that need to be "up to code" and it's not just internals like electrical, etc. Stairways, railings, heights, room sizes, construction technique, etc are all affected by this. And we continue to add more and more code for things like efficiency, etc. And this isn't a bad thing - code means we have safer buildings and when someone buys a building there's at least a minimum standard that has been met.
So this means you are going to build in ways that are proven and will be up to code. Which leads to the second thing.
Construction materials have become standardized over the last 100 years or so. We went from a world where things were crafted to a world where things are manufactured and installed. Buildings are made mainly of commoditized parts and materials today which makes a lot of things look simplistic and similar. This is good in many ways because when a craftsman was hand carving things they were not very productive and it cost a lot of money. Today, a worker is amazingly productive since their tools are powered and their materials are pre-fabricated and just need to be cut to size.
Lastly, buildings are far more complex today than 100+ years ago. Lots more electrical and plumbing, complex HVAC, insulation requirements, etc. These things were not considered long ago. You had carpentry and maybe a bit of plumbing and some basic electrical.
You can still get really nice custom work done and it's unbelievable expensive.