Because if you are a musician you are supposed to be able to recognize that stuff when you hear it. If you don't know a major scale from a minor scale or a pentatonic or a mixolydian, take some theory classes. It will truly help your song writing when you are looking for something that doesn't sound cookie cutter.

Chances are you already PLAY all the different scales. You just don't know the theory behind them to know you are playing them.

Think about chord structure for a minute and how much voicing means. If you play a Dm (with the A on the bottom) with your right hand over a B bass, you are playing a Bm7b5 and you don't know it. An F chord on your right hand over a G in the bass is an F9 because G is the 9th note on the F scale. I am sure you have played that chord. Maybe you didn't know it was an F9. Stuff like that is examples of how different things become second nature by virtue of hearing them and finding them on the keyboard and not knowing what they are. You don't have to know what they are, but it helps you write (not better, but often) faster if you do. If you know the theory, your writing will seem easier when you are looking for chords.

So the non-long winded answer is "Because that's the kind of thing musicians are supposed to know."