Because most of our public leaders have a liberal arts rather than a technical background. I want such people to have at least some inkling of the ever growing technical nature of the world that they are legislating or administering.
Because physics and chemistry without calculus are rote memorization of the worst kind, appearing to be a bunch of ad hoc rules that we nerdy scientists pulled out of an unnamed body orifice. Calculus suddenly makes all of those ad hoc rules make sense.
Calculus is not a torture device. It is a very simple, and very teachable, mathematical concept. With this simple concept, many different topics ranging from finance to population dynamics to planetary orbits becomes easier to understand. Calculus is not hard, at least not conceptually.
That said, many schools intentionally do turn the mainline calculus and introductory physics into torture devices. These are the canonical bust-out courses for the physical sciences and engineering. Because calculus and physics are useful in the life sciences, many schools also offer less intensive calculus and non-degree physics classes aimed at those students. These classes aren't really dumbed down versions; they just aren't intentionally made to be difficult. Those courses would also be appropriate for liberal arts majors. There is a difference between making a subject not so difficult and dumbing it down to the point of being meaningless.