They learned that stuff because they are either basic methods in the field that have demonstrated value, or what they're actually doing research on.
The syntax buys them power that they wouldn't otherwise have, and they can smell it.
S-expressions? Not so much - the notation is more or less and arbitrary choice unless you're going to leverage the metalanguage capabilities to the hilt, and there are many other programming approaches available.
A mathematician who really wanted to jump into the deep end with programming syntax/structure would much more likely pick up Haskell (categories, yay!) or AVL rather than Lisp.
We could sit here and grumble about how silly it is that people cannot overcome such an irrational response (indeed I often do), but that won't change the fact that people have these responses. If we did that, we'd be doing the same thing that GUI designers did in the 90's. "Why, the widget for $x is right here. Any reasonable user will RTFM, know that $x can be checked there, and behave perfectly rationally." (And it happened just so, as decreed from developer's rolling chair, right?) Of course, we know now how misguided this is as a method for UX design: all users are in a sense irrational, and systems need redundant strategies for saving the user from themselves.
The similarity here is that we cannot will away intellectual blemishes in users, we can only accommodate them. That is Lisp's challenge if it is ever to get big. For GUI's, the solution was to take seriously users' need for visually attractive interfaces, redundant display of information, and guardrails/handholding for actions commensurate with their risk, to name a few strategies. For Lisp, it's hard to know what might be the right approach. As klibertp writes in a sibling comment, maybe the solution needs to come from outside the language itself.