The use of monads is a side-effect (ha!) of committing to purity throughout a language, and that's what FP is being equated to: pure statically-typed FP.

(You can argue about how justified that is, of course. I'm not going into that, but you might want to see what John Carmack has to say[0]. No, he doesn't end by saying "we should all convert to the church of Haskell now", but he does talk about how large-scale game programming refactors are made easier when you're working with no (or very disciplined) side effects.)

Monads are not the only way to deal with effects while keeping purity, although they were the first discovered and so on: algebraic effect systems as in Koka[1] (and as simulated in Idris or PureScript) are another alternative. Koka infers effects, so it's probably easier for a C-family programmer to pick up (I know little about it, though).

[0]: https://www.youtube.com/watch?v=1PhArSujR_A

[1]: https://www.microsoft.com/en-us/research/project/koka/