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.
> And hopefully you made a USB install stick first, since they don't ship with install media anymore.
As if anybody actually does that any more? I just bought a new ThinkPad and it didn't even come with a manual, just a little sheet of paper telling me where to find the PDFs.
I extended the life of an iMac by booting from an external SSD over Thunderbolt. I couldn't face the replacement process at home. Performance is very good.