Imagine pushing a pastry cutter into your hand, then doing the same with a disc of solid material the same diameter…..
Obviously, however the force required to force a bar end plug into the tube isn’t that great* whereas the force required to push this through your body is pretty major. Obviously screw on end caps solve that but I’m not convinced that fairly soft plastic caps offer much protections, especially not the really flimsy OEM ones you sometimes get.
*I’ve done it just laying the bike down on the ground before.
So, don’t use flimsy ones? If you’ve managed to do that just laying a bike on the ground before, then they must have been really rubbish plugs or poorly fitting. Or you have a really heavy bike! 😉
I’ve never managed to force a bar end plug into a bar.