Rarely do teams bracket a TE 5 yards off the line of scrimmage. Kelce averaging 8 yards be catch is essentially a running back averaging 8 yards per carry. You do that give up those kind of yards you lose.
Notice that Kelce is extremely bendable and gets his center of gravity low to the ground while making those cuts and often stays low to plow for yards. He’s 6’5” but presenting a 4’ target on some of those cuts and dives.