> Because schools aren't about to let students use phones/laptops during exams
I was totally blown away when I entered a US high school only to find that math class was basically teaching you how to use the calculator, not how to do that math.
This was not my experience in a US high school. We weren't permitted to use calculators until we demonstrated paper-and-pencil proficiency with a topic, and we always had to show each step of our work.
But education is mostly regulated by state and local authorities in the US, so YMMV.
School quality in the US is very location dependent, as public schools are largely funded by property taxes. I also went to a public east coast high school, and calculator use was mixed. Some exams allowed calculators, others not. Often the exam was designed such that it was testing your knowledge of a concept more than the numerically correct answer, showing work was required. So a calculator was useful for checking answers but correct answers with no work would get you a 20% at best.
I was totally blown away when I entered a US high school only to find that math class was basically teaching you how to use the calculator, not how to do that math.