Even among the engineering-oriented lower division math courses, material in differential equations courses is both antiquated and presented like a laundry list. This essay is more than 20 years old now, but the complaints are basically as relevant today.[1]

[1] https://web.williams.edu/Mathematics/lg5/Rota.pdf

Good scientists are often terrible teachers.

That and people default to being scared of mathematics rather than seeing difficulty as proof of it being rewarding to learn.

Both. They are taught, generally speaking, by giving a series of techniques to solve problems without the underlying mathematics (i.e why those techniques work on those problems). Calculus generally takes some time to grok, which typically doesn't have unless your really into it. Think how many people fail to grasp exponential growth (the simplest ODE).

The concept of a differential equation is not hard. Solving differential equations can range from trivial to non-trivial.

Learning hard things is difficult for anyone, even smart people