This is going to sound extremely cynical, but I believe that there's another reason why people enjoy coming up with ideas while in the shower or driving or taking a walk or...etc.
When you're doing one of those things, you can't actually do any work towards implementing the idea--but you can imagine all of the things that you're going to do as soon as you're able to (which, of course, you won't do).
In the shower there is no pressure to actually do something. You don't come up with ideas at your computer because you'd be confronted with the fact that if you really wanted to, you could begin implementing one of those ideas immediately.
When you're doing one of those things, you can't actually do any work towards implementing the idea--but you can imagine all of the things that you're going to do as soon as you're able to (which, of course, you won't do).
In the shower there is no pressure to actually do something. You don't come up with ideas at your computer because you'd be confronted with the fact that if you really wanted to, you could begin implementing one of those ideas immediately.