There is a sizable difference between PCI-e SSD and HD form factor SSD
One runs a PCIe speeds, the other at SATA speed.
Memory for PCIe is much more expensive.
Of course, but Apple still sells a ton machine with spinning hard drives for premium prices
My argument is their offerings are old, and they want a premium for that old technology
Of course in the $7,000 Mac Pro desktop they use the best of the best, but then again you are paying super top dollar so you should get the best of the best
BTW the Intel PCIe Enterprise class SSD can be had on New Egg for under $800, just priced one out the other day