Engineers care about arithmetic. Carpenters do too. Any number of other creative endeavors require (or, at least, are dramatically improved) by the ability to make basic calculations (even if approximate) quickly in your head.

Arithmetic is the "write one sentence" of composition. The ability to think through a series of calculations with real-world context and consequences is the 5-paragraph essay. If you're not competent with the basics, you won't be able to accomplish the more advanced skill. Being tied to a calculator (not merely using, but being unable to not use) takes away intellectual agency in the same way as an LLM-generated essay (though, I'll agree, to a lesser degree).