Then they've been trained incorrectly by terrible coaches. Wrap up the legs and a ball carrier can't run, period, I don't care if you're 1000 pounds, legs wrapped up you go down. Someone mentioned they faced running backs 70-80 pounds heavier, I did too, had a buddy who was 250lbs and people used to bounce off of him when they tackled him incorrectly, when I tackled him, and I was 170, he went down. Why, because I was taught the correct form tackle when I was 7, and practiced it over and over for years. Can't say I got myself in position to make every tackle, but once I wrapped my arms around your legs, you went down.
Same goes for blocking, my Dad still has a picture of me making a textbook block on a kid from when I was 9 (I'm 52). Why was I able to do it, I was taught the proper technique, over and over in practice for years. You'd think these guys would know how to preform a proper tackle by now.