So I can somewhat speak to this, as I actually did this for a while c. 2021 and 2022.
The issue (apart from my Mac mini still having the old, bigger form factor back then) is that this requires a full shutdown (obviously), and that is more friction than opening a closed laptop lid. It just takes a moment for every app and background service etc. to settle back in after, and depending on how your brain works, you might not like that a whole lot.
It absolutely is a cool feeling to carry a pretty mighty desktop in the backpack, though.
I think it's actually good to have a forced reboot from time to time, but then I'm not very disciplined in closing stuff I'm no-longer using! Especially like docker containers I was using months ago, that I forgot to shutdown.
You don't get the option in the UI: there's just "Sleep", which on a desktop Mac seems to suspend to RAM only. At least, it seems to on mine, as I found out when I unplugged it.
(Apparently the laptops do a suspend to disk depending on battery remaining, so the OS does support it. It'd be nice to have it available! The disk space required could become annoying if you only bought the 1 TB SSD for your fancy 512 GB Mac though.)
At that point you lose too much convenience. Maybe if someone built a mini UPS with at least 10000Ah capacity, USB-C 4 pass-through and the right form factor to permanently attach it on top of the mini.
The issue (apart from my Mac mini still having the old, bigger form factor back then) is that this requires a full shutdown (obviously), and that is more friction than opening a closed laptop lid. It just takes a moment for every app and background service etc. to settle back in after, and depending on how your brain works, you might not like that a whole lot.
It absolutely is a cool feeling to carry a pretty mighty desktop in the backpack, though.