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I've met software developers who didn't know how to use Excel properly (in the sense of not even knowing they could use formulas).

I think that's very much for the reason you state: they "won’t go out of their way to learn a software they don’t even know they will use".

It's not just a "common man" thing, it's an everybody thing - there's just too much stuff and not enough time to learn it all, so even software developers might never find themselves in a situation were they have to understand Excel enough to know such simple things as how to use functions in the cells, how to use references to other cells or how to make some references be relative to a cell's position and other absolute.

Mind you, they'll probably learn it way faster than "common" people simply because so much of its advanced usage follows "programmer logic", but that still requires them to be forced to actually use it long enough and often enough that they put the effort into learning it.