specifically mountain bikes.

• They use standard parts that you can find in an any abandoned bike store or department store

"Standard" in that there are multiple incompatible "standards" for wheels, hubs, chains, cranks, chainrings, shifters, derailleurs, derailleur hangers, brake mounts, brake fluid, handlebars, seat posts and probably 50 other things I've forgotten.

And department store bikes ignore most of those standards and do whatever is cheap.