I can write drivers, but it would take me a long time (I've never written driver so factor in a learning curve). Or I can select parts that linux has drivers for - but again it would take me time. I can assemble a computer, but it will take me time. My time is not free.

It takes at most 4 hours to put a computer together even if you're new.

If your time is worth more than 500$ an hour congrats. Then you can have a shop put it together for 150$ to 200$.

What are you going to do when it's time to upgrade? Spend a few thousand on a new CPU and keep going or throw it out and buy a new pre built?

The System76 machine bested Phoronix's DIY setup. So clearly they aren't equivalent.

I don't see how this is possible assuming all the parts are identical.

If I'm too afraid to build my own computer I'll buy a Mac

Components are not identical. The cpu is, but small things like the case is different and that can make a difference (cooling)

I am not paying for the install but for their work.

Try installing Windows 7 on a new laptop or Windows 11 on a Chromebook some time. Lots of stuff will not quite work right.

As with a lot of things, you can outsource that tinkering to someone else. Seems like a sane business model to me.