Not if the drive is soldered to the motherboard..
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.
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.
Hell, I'm sure they could make it a tool-less operation if they wanted to.