Just because a person is obese doesn't mean they're not a world-class athlete. If these guys were trim, they couldn't succeed at their job. And, some people are just fat. they have fat parents, and they're fat. it is what it is.
edit - and just because they're fat doesn't mean they don't have lots of muscle either. you don't want a guy who's 320 pounds with only 7% body fat on your line either. The fact is, with all that muscle which needs oxygen, they wouldn't have enough wind to perform as well. in a very real sense, the guy i described would be actually be far less athletic than most of the fat linemen in this league. All that muscle takes lots of oxygen. that's why you hear so many football players getting made fun of by other football players because of their "popcorn muscles". it's because big puffed up muscles that don't help with specific football motions are actually detrimental to aerobic output and athletic performance.
so, in response to your question, Nonstop, "That makes sense, but wouldn't they prefer that mass to comprise muscle rather than fat?" In many cases, the answer would be no. not always. and i'm not saying you don't want your players as strong as possible. what i'm saying is that muscle mass isn't always "better" than fat mass for linemen.