They should have followed Silverstone Alta gm1 layout which is much better interms of air sirculation and cable management. Buttom IO is not a good idea.
I haven't built in that one, and I don't generally have a problem with the bottom IO, but this case does make some strange airflow choices. It's a real head scratcher.
My friend, the simple answer is because whether I put an M.2 SSD in this computer or not would make zero difference to the point of this video; which is to build a computer in the Corsair 2000D. That said, I understand why you raised that point. M.2 SSDs are generally faster, even though they can be slightly higher priced. However, it's important to recognize that even a low capacity SATA SSD like the one I used can be more than good enough for a variety of uses.