Because they were 6'2 Wide Receivers in high school who realized they did not have enough quick twitch muscle fibers if they wanted to pursue the dream of the NFL, so the ones that gain 50lbs who don't mind being a virtual ole school fullback , H-Back or glorified offensive tackle may make it. If they can deal with run blocking.
The Julius Peppers ( I know he a def end), Tony Gonzales', Antonio Gates' and Travis Kelce's, all could hoop but only as non jumper having 4s, but they could crash boards & that indicates good mitts and hand eye, but that non ideal NBA height is a no-no , which leads them for a natural fit as a elite tight end.
Its weird fit sometimes, So take a athlete now, like LSU's Drake Davis, he was an "elite" soccer player who can really dunk and run a 4.3, but he is averge maybe mediocre wr, who could likely be the world's best lacrosse player, but ain't no money in it so generally

we don't fools wit it.