Some people build guitars other people play them. They are related disciplines and guitar builders know how to play a guitar but it's not their primary pursuit. Biking is no different no? Some people are interested in how to setup and combine components in an optimal way. Others want to ride.
indeed, building vs using is an entirely valid hobby. some people like to repair cars for fun, or restore old ones. i knew someone who restored old steam engines. or people who like to restore/build computers from used parts and give them to kids who need one.
i built my own bike when i was young, but mainly because it was cheaper to get a quality bike that way and easier than saving up the money for a brand new bike. in the end riding is more interesting, so now that i can afford it, i just buy what i need.
