Many people doing OO have trouble structuring their classes, splitting functionalities and responsibilities properly.

OO kind of lets you do anything you want, allowing you to store data anywhere and having any kind of flow you want. It's very easy to do something that "just works", and while there is a lot of value in that, you can keep bad habits for very long.

FP is a lot more constraining, and will force you to adapt how you think to fit said constraints.

Having to think differently offers a different perspective on code architecture, that will eventually give you ideas applicable beyond FP languages.