Think of cars. Cars replaced horses. Ever tried to design or build your own car? It's hard. Expensive too. My attempt consisted of retrofitting the recumbant shown below with an engine and gas tank, and a fiberglass/canvas shell. We're talking $ thousands $ for a simple go-cart or motorized buggy. And it would need repairs at some point, that costs more money. Replacement parts are not home-made (fabricating machines/tools are too expensive, and require repairs & replacement themselves eventually), you have to buy them from somewhere - they are mass produced.

A horse just needs grass to eat and a field to frollick and gallop around. So simple.