Why aren't 90% of the raiders armed with them?

Because people are finding the ones the raiders threw down to pick up legitimate weapons.

I dont even think in any fallout series any enemies carry BB gun

I don't think in any fallout series game, I found so many BB Guns...

so far I only got 3, and sold them all

I'm lucky if I only find 3 a trip. My last guy brought back 8 out of his 57 items he had.

The real question is, why do raiders not drop stuff? They come in with weapons don't they?

and some caps right? since they are raiders and they raid ppl for caps