I never understood why people say" never bet on the river if the only hand that will call you, can beat you".

I mean, if 2 of you have got that far, and you're certain your hand is better then why not bet out?

Sure, you can walk into a more expensve beating every once in a while than if you'd just checked but it's very likely the villain would bet at you anyway.

It's also common to read "don't bet on the river unless the only way you can win is by a bluff". But this goes back to the above points. If it's ok for a bluff to be bet then why not a solid hand? And if you shouldn't bet 'cause you can walk into a beating then likewise with bluffing on the river?

Maybe it means only bet if you cannot be beaten at all but short of landing quads, most hands are beatable. Eg: only today I walked my flopped nut flush into a completely unreadable full house on the river.