May I ask a question: why does Windows 7 show UAC dialogs at all, if you are signed in as admin with default UAC level?

I mean there are clearly ways around the dialog. Whats the deal with the dialog? Why is it shown? Why not elevate all applications? I mean malware could silently elevate (is it Vista, is it 7). And all the other applications show the dialog, which is broadly hated eeem unliked by people...