Why does a football team QB kneel when they possess a commanding lead?

I can understand if the clock time is low and the high scoring team wants to just end the game and not risk the ball, but I see teams who command a huge lead (8 points or more), yet will still do it! Why dont they attempt a long pass to possibly put even more points on the scoreboard and to possibly add to a QB and the receiver’s ratings?

Its classless. If you know your going to win why run up the score? Why risk injury’s to just run up the score? Thats just stupid.