Because they play way more football than most of the rest of the country. As someone else said (in this thread or another thread, not sure), in the south you’ve got football practices in the spring as well as the fall. Football is the lifeblood of the south. EVERYONE plays, all the time. The kids get to develop their skills a lot more and the talent rises to the top. The more you play against top talent, the more developed and skilled you become. The coaches aren’t just biology teachers doing double duty as coaches as well.
Overall, it all matters. Yes, athleticism can’t be taught, but if you are working on football from the age of 6 on, with all kinds of coaching and development, you are going to find more talent in those areas than other places.