There are downsides to FP as well. I am not advocating one over the other. But there is a concrete theoretical reason why FP is more modular, reuseable and organized than OOP code.

Smalltalk is possibly the only OOP language that lets objects be compose-able and modular. Check out Pharo if you're interested. If you learn smalltalk well enough, you could apply its principles to traditional OOP languages and gain the modularity benefits.