Clarification: If you played with cards face up no one would ever make a mistake. You could calculate the odds and the math would be perfect. The goal of poker is to make an opponent make a mathematically incorrect play.
A simple example is a rock paper scissors tournament. The nash equilibrium says you must always simply pick each with 1/3 chance. But if you play this you are almost gauranteed to finish in the middle of the pack. The winner is going to be someone who can take advantage of other players. There used to be a rps AI tournament years ago (UofAlberta I think), and this is why they wouldn’t take submissions that only played nash equilibrium play.
But you are right, in that usually the first step in getting good at poker is to simply play the strength of your cards based on your position. And this becomes more amd more important the higher the stakes since players tend to be better there.
It's also true against someone who is playing an exploitative strategy.
Even on-line heads-up challenges are played on 2 tables in order to not be boring and that is 500-600+ hands per hour
(cue the often useless, recursive aspects of pop-culture game theory)