[bzzzt]

Of course it was a decision by Apple, but I'm not really sure it's that 'tricky'.

There are multiple ways to upgrade storage: since Macs retain the value quite a long time compared to PCs, if you really want everything integrated and need you can sell your Mac and just buy a bigger one. Then there are several options for external storage (from USB stick or SSD to NAS to Thunderbolt disk arrays).

The integrated 'root disk' also has advantages: Apple controls the entire stack including drive firmware so it's guaranteed not to have nasty surprises on Mac like some PC SSDs I've been bitten by in the past. Also, performance is uniform and it's impossible for a drive to fail or shake loose due to a bad connector because there isn't one.