At a certain scale, it's impossible to directly implement anything anymore. You can have tremendous impact but the more layers in between what's in your head and what's produced can only increase. Bill Gates wrote code that shipping in Microsoft products until the late 1980s[0]. At a certain point, he had to trade off having a bigger and bigger impact by managing the implementers instead of implementing himself.
Except somewhere along the line when you become the guy managing ten engineering departments with ten people each that help ten people write code, then you're not doing anything anymore. You're just enabling people to do things. And it becomes more and more likely they may do things in a way that's not in your head. So you trade implementation for impact, but then eventually the impact you get may not be that impactful to you because it's no longer what you did but what other people did.
In a startup, you have the best of both worlds. Impact and implementation. Eventually you fail because you didn't make enough of an impact, or you stop implementing because it's impossible for you to scale with the impact you're making. But I imagine the ride to that point is extremely fulfilling.
[0] http://www.microsoft.com/presspass/exec/billg/speeches/1997/...