Because that's not a 'Super' search: you can't specify what sections you want to search, or which authors you want to (in|ex)clude e.t.c. And more importantly you can't exclude replies (if you use the normal search box).
Case in point: before starting this thread, I did some research by doing a Super Search for PM Discussion nodes whose title contained 'Super Search' (I didn't find any questions similar to this one). But if you put 'Super Search' (without the quotes) into the normal search box, you will get the Super Search page itself instead of nodes with 'Super Search' in the title.