Because you need multiple types of Torx security screwdrivers and have to literally disassemble the thing 100% (including removing the HSF and pulling out multiple tiny ribbon cables) to get the hard drive out; and it completely voids your warranty to do so. And hopefully you made a USB install stick first, since they don't ship with install media anymore.

I'm sure the next iteration of the Mini will go to 32GB/64GB(+$100)/128GB(+$300) M.2 SSD options, all conveniently soldered onto the mainboard as well.