Mech designers weren't supposed to be mech pilots.
For many mech designers, it simply wasn't possible to pilot a mech due to their genetic aptitude. In the rare case a potentate decided to become a mech designer, their piloting skills were often far too poor to be of use on the battlefield.
It took too much time and dedication to become good in one profession. Most people in the mech community only possessed a talent in one or the other. Those who excelled in mech piloting would not possess the traits that helped them become a mech designer.
All in all, it was a matter of efficiency. Rather than train a mech pilot to minor in mech design or vica versa, it was better to dedicate their time to their best profession and see it as far as possible.
Only those who put their entire time to a single profession were the most likeliest to surpass the extraordinary threshold and beyond!