With thin laptops you can maybe justify that as a necessary compromise for the form factor, but there's no good reason the Mac Studio shouldn't have a hatch on the bottom with two or three M.2 2280 slots inside.
You can put your own PCIe SSDs in the Mac Pro, but that's so wildly marked up compared to an equivalently specced Mac Studio that it probably cancels out any savings.
I remain flabbergasted that there's no model of Mac Mini or Studio which has M.2 slots. I believe the Doctrine is that Thunderbolt is fast enough that YAGNI but that's absurd given the amount of open space inside those things. If you look at a picture of the motherboard[0] it's dead obvious that there's room for one M.2, maybe two if they cared about it.
Thunderbolt is decently fast but the enclosures don't come cheap, it's an extra $100 or so per drive you want to hook up. The enclosure could easily cost as much as the drive you put in it nowadays.
There is: Not wanting to support end-user modified products.
Apple is never shorthanded on white knights, but more generally I can see why companies might opt to lock out customer modifications under practical pretexts.
To USB you mean? The cheap ones only do 10gbps, not even a quarter of the speed a PCIe4 drive is capable of. 40gbps adapters are up in the ~$100 range.
Oh, that type of adapter. Those work but they'll only fit in the Mac Pro, which has a $3000 premium over a Mac Studio with exactly the same specs, so you're unlikely to come out ahead there.
Sorry, I realized I misread your previous comment. I thought you were saying the markup on a PCIe drive for the Mac Pro was too high. Now I understand you were referring to the markup on the Mac Pro itself.
You can put your own PCIe SSDs in the Mac Pro, but that's so wildly marked up compared to an equivalently specced Mac Studio that it probably cancels out any savings.