Apple have access to innumerable amounts of user data that probably tells them only a small percentage of users ever bother turning them off. Indeed keeping them on over night allows the system to perform maintenance tasks and IT professionals to push out corporate updates all whilst using barely any power at all.

There is also the interior design to think of. Whilst I would have liked to have seen Apple use the logo on the top as a capacitive power switch this would have added to the complexity of an already tiny device. Looking at the thermal stack inside there isn't anywhere else they could have put the power switch.