Because it’s so easy to find a crippled old guy on dialysis in a Pakistan home. On the other hand, a guy who can move, and who has no problem dressing up well enough to not be distinct as a field commander, combine that with limited physical troop presence and the depth into the mountains, and well, getting these guys is like trying to hit a needle in a haystack with a .22 pistol from 40 feet away. It appears that the same means of hiding from hits by fighter jets or cruise missiles works pretty well in hiding from UAVs too.
Same with Omar, nobody can find him either.