1, I think (especially in football's case) you're mistaking bulk for body fitness. Some of those 380 lb monsters in the NFL can still run at inhuman speeds and put their bodies through incredible stress to keep the size up. A large majority of them, upon retiring, will quickly lose weight because they're no longer in need of the bulk.

In baseball, you'll have very few truly out of shape players (a la Bartolo Colon) in the modern game, and they're usually relief pitchers whose entire job is to throw hard for a very short period of time. The players tailor their bodies to their positions, so catchers tend to be bulky, outfielders athletic, infielders quick, etc. It's a very specialized sport based on position.

2, you can buy MLB jerseys on Amazon, they're just not the official Nike. I don't know about world sports, but generally everything here goes with demand and branding. Just adding that swoosh probably doubles the price.