I love this framing. I have a lot of ideas about why this is true, but the best (IMO) pat explanation I've been able to come up with is:

1. Much of constructing software is math of a sort. In math, you write proofs that say why X is true. In software, you write code that guarantees X will be true (e.g. "the backend assumed a user ID was always available, but now that we have service accounts, we need to change the access control code so that a sensible view is still returned")

2. Math is hard, and most of the work is invisible thinking. If you asked a mathematician for an estimate on when the Reimann hypothesis will be proved, they would laugh at you. Our problems are generally easier, but they can still be hard. And as in math, they're sometimes much harder than you expect (e.g. Fermat's Last Theorem. "Why was it so hard? Did you talk to Fermat? He left? But he said it would be easy!")