Most of commercial developers have:
- very little spare time to play around, so it's most economical to buy things if it saves times
- enough income to be able to buy their equipment tax free.

Quite a lot of hobbyists have almost no money at all to spend on their hobby. But they may sometimes have almost infinite amounts of time.

When in school, I did not have much money to spend on components. So I did desolder components and reused. Recommendable? Of course not. But it allowed me to build things I could afford to build any other way - or at least it allowed me to build them quicker.

A Z80 might cost $20 and with all the external components needed, it took a long time to get all parts to build something. The mass-market prices for components are way lower now, but the local stores with electronics components have gone away. So it some situations, it's just as expensive now to get components to a private project.

Besides - it's way more fun to have built everything from scratch, than just dropping some software into a ready-to-use dev-kit. Of course, I do prefer to use a dev-kit now, since I'm more interested in the final function than the path to get there.

But it really is important to notice the very broad span of availability and economical resources involved.