The worst penalty for somehow failing to successfully manage a wall-to-wall pull is someone saying something nasty about you on these forums or reddit. That's it. There is no experience loss. The whole dungeon doesn't reset. You don't lose your expert roulette bonus.

There is nothing to lose, so players are going to pull wall-to-wall even if it was harder. Part of this is the developer's choice and part of it is how players play the game... coupled with developers then catering to what players want.