Eh, I mean it's not on the SATs, but why isn't it? In Canada, we had a similar calculus based curriculum up to the first year of university. A little bit of linear algebra thrown into the mix. Why is that? Well you need calculus to do any form of engineering, physics, certain domains of chem/bio, stats, certain domains of economics, etc etc. Math in society is first and foremost a tool. I say this as a person who majored in Pure Math and focused on algebra and number theory. For the vast majority of students, it truly is about the practicality. Math just has the layer of abstraction that makes it hard to enjoy without deliberate framing unlike the sciences or humanities.

Many people dismiss mathematics because they aren't interested in it. I definitely wasn't interested in doing dozens and dozens of error-prone problem sets that mainly boiled down to performing arithmetic. I don't need to do that.

My number theory professor was a brilliant mind, someone who had spent lots of time at the Institute for Advanced Study, and he absolutely sucked at adding/subtracting/multiplying numbers. And that was something he freely admitted. It wasn't important to his work, and it isn't important to mine.

Calculus isn't on the standard SAT or even the Math Subject II tests [0], at least not in the early 2000s.

They taught us calculus because it's a prereq for engineering and physics and that's been important since the space race.

[0] https://en.wikipedia.org/wiki/SAT_Subject_Tests