You know, this is a good question.

Googling it would ruin the fun, but I’d assume that it’s an evolutionary thing so you don’t decide to go to sleep in an unsafe situation and get yourself eaten.

Or dance to sleep

But we (and other animals) don’t just fall asleep involuntarily? We do decide to (go to) sleep. I know I missed your question with this take.

Maybe it’s a bit like when we try to focus to learn something. We have to create a situation which supports that desired brain activity, and control our attention. In both cases, we want to minimize distractions.

So I guess one answer to your question is: Because sleep is not a simple activity like watching TV or riding a bike, which we could switch to in an instant. Sleep is a state of mind (and much more physiologically), which requires a certain environment, relaxation and time to transition.