The FTOP and calling on the river

[Ghazban]

Assuming there are only two people in the pot on the final betting round and one of them bets, isn't it true that calling is always a mistake according to the FTOP? If the caller could see his opponents cards, he'd raise if he was ahead and fold if he was behind.

Obviously, we don't get to see our opponents' hands for certain so calling is often better than raising or folding versus an opponent's entire range of hands, but that's not really what I'm getting at.

Its just really really odd to me that calling on the end headsup is always suboptimal yet it happens on almost every hand.