My understanding is that on Desktops, the default is for Sleep to do 'Hybrid Sleep' - prepare for a hibernate, but sleep instead; if you have a power failure while the computer is sleeping, you should see it waking from. The hibernate when you turn back on, instead of booting from scratch.

I know that this was the case at some point when I checked during the Windows 8 years, I don't know if this is still true.

My guess is the same reason modern UIs suck: too complex for the average non-technical user to handle. They probably got tons of support requests asking what the heck the difference between "sleep" and "hibernate" was, and that's a hard question to answer if the person asking has no idea what RAM is...