Computers are state machines. The problem is that the initial condition the computer starts with (which is, literally, instant) is not very useful to you and me. (Even after the first electron moves).

What's useful to you and me is many millions of steps further down that state machine. The easiest way for developers to describe that state is define it in programs, and those programs will always take non-zero time to run.

Now you might say: "fine, but can't you cache the 'first-usable-state', and start there". And you could, and it does, it's (roughly) what resume from sleep is. (and which is why it's not very fair, I think, to "exclude" resume from a discussion of improving boot times).