Why don’t you just get a multi-slot PCIe box?

I could.

It would be even cooler if that box was also housing my computer and powered by the same power supply.

And then the PCIe lanes could just run to the CPU/SoC instead of having to be wrapped in Thunderbolt.

And here I am with a gaming PC that has absolutely nothing in the PCIe card slots except for a graphics card. 1 out of 5 slots filled!

The truth of the matter is that there's basically no hardware on the market that actually depends on PCIe bandwidth besides graphics cards.

Furthermore, us computer nerds don't like to hear it, but making someone open up a computer is a barrier for many customers.

E.g., I'm selling a computer to a video professional and they want a processing appliance that connects to a computer to help with their video workflow.

Is that video professional also skilled in computer subjects like opening up computers? Probably not! A nice box that you plug in to Thunderbolt is way simpler.

Bonus points, you can plug that box into any other computer without taking your whole computer apart.