My job involves 80% rote memorization including a lot of mental math. No being able to calculate simple stuff would slow me down at least 2X. It's a high-pay job.
What? As a software engineer, I'm constantly doing mental math to determine things like "how much memory will this take up when the input size is X". Sure, Big O notation is nice for limits, but in real life, there are typical input ranges, and you have to do the calculation. Can I use a calculator? Maybe sure, but why would I?