Calculators shouldn’t be allowed on any mathematics examination. With limited exceptions, high-school mathematics and beyond is not really about arithmetic or numerical solutions, but about demonstrating understanding, which you don’t need a calculator for.

Questions on exams that use numerics also put people with dyscalculia at a disadvantage even if they have great ability at differential and integral calculus and other symbolic operations.

But mainly because it’s the best way to sneak-in crib notes past the exam invigilators. Especially in the US where everyone uses TI graphical calculators (yay for regulatory capture).