This is not a rule. A lot of the players don't use much math--basic pot odds. They develop a feel for when they're making the right play, and openly admit to being "feel" players.
I do believe they would be very good at doing on-the-spot calculations in their head, they're often just not very trained in math.
Anyhow, surprisingly little in poker is math-dependent, particularly in normal ten-handed games. The cognitive load is mostly about factoring in thousands of variables, past hands, and potential hand combinations of your opponents.
There used to be prominent winning players who were unaware of the math behind the game, but the theory nowadays is they mostly just lucked into naturally playing in a style that happened to be better than the field during the period of time they played (i.e., years ago back when the game was less sophisticated).
Nowadays, those players are often the weak players who the players more skilled at theory can comfortably beat.
There's an eerie parallel to the early stock traders here.
Your statement is true about the "feel" players, but even those players will perform EV analysis away from the table because by estimating percentages of certain tendencies you can figure out what the right play is in tricky situations, and over the long run that ends up saving more money than anything else
Of course feel players are doing EV analysis, but that's 99% about estimating hand ranges properly. The math is certainly the most simplistic part of the analysis.
I do believe they would be very good at doing on-the-spot calculations in their head, they're often just not very trained in math.
Anyhow, surprisingly little in poker is math-dependent, particularly in normal ten-handed games. The cognitive load is mostly about factoring in thousands of variables, past hands, and potential hand combinations of your opponents.