I assume by "do maths" you mean doing simple calculations, like adding a bunch of small numbers, in one's head. That's because in many situations it's more convenient to do so, than using a calculator. So the skill is preserved / practiced, because a calculator is too cumbersome to use. The skills of most people settle at the equilibrium where it takes the same effort to take out the calculator and focus on typing, as it would to strain the brain doing it without a calculator.
> We have people who can still spell after the introduction of spell check.
When using spell check to fix your document, you automatically learn to spell. Your skills improve by using the tool. A better analogy to AI would be an email client with a "Fix all and send"-button, where you never look at the output of the spell checker.
Both require manual "labor" which leads to learning.
Also to note. Calculators merely solve intermediary steps. LLMs are increasingly designed to do a one shot full blown work. Longer context, deep thinking, agentic loops.
In practice, what this means is that you can read some subject many times, but you would still struggle to reproduce the content by yourself. That is why, when learning, it is not sufficient to just read the material several times.
Of course there are people who do maths after the introduction of the calculator Just like there are more people who program after the introduction of the electronic computer.
Arithmetics is a very, very small subset of math.