Canned firmwares solve a lot of bootstrapping problems, and there are spoon-feed tutorials to get a working PCB, but you run into a lot of real-world constraints soon.

While loose wiring allows for custom experimentation of some designs, ergonomic 3D shapes, it also tends to increase complexity of assembly and risk of things going wrong (shorts and loose connections) It's also a mess if you want custom lighting. So you tend to stick to PCB-based designs for anything beyond one-off projects.

Commercial quality mounting plates and cases are more of an impediment than the electronics. Yeah, you can make a science fair project, but for something you're using 12 hours a day for years, people want a point of pride. Want one 104-key switch mounting plate cut? It's probably going to be over $100. Bespoke CNC milled cases can be over $500. Since there are commercial offerings now for many popular sizes, it's easier to say "I'll make a slight riff on the existing 60% size, so people can use a cheap case, than to try a zesty new layout that means a $400 admission charge."

Note this is from an enthusiast short-run-of-kits perspective. If you're going for commercial production, then you have issues with sourcing and manufacturers. We're now into year five of waiting for the Matias corporation to get new keycap designs they promised to market.