I don't use calculators for most math because punching it in is slower than doing it in my head -- especially for fermi calculations. I will reach for a calculator when it makes sense, but because I don't use a calculator for everything, the number of places where I'm faster than a calculator grows over time. It's not particularly intentional, it just shook out that way.

And I hated mental math exercises as a kid.

I do not trust myself, so even if I know how to do mental math, I still use my computer or a calculator just to be sure I got it correct. OCD? Lack of self-trust? No clue.