No one likes bad calls, but the refs make some incredibly accurate calls in real time. To your point, they see things I'd never see in regular time. Yeah they make mistakes, that's part of the game and I think it evens out over time. Now there are some horrendous calls, like the missed PI call against the Rams that sent them to the SB instead of the Saints, but game changing bad call like that are rare. (And in the Rams case, maybe bad karma as they lost in the SB anyway.) They tried allowing challenges on PI and it just didn't work and slowed the game down too much.