The problem is that software design in general is an incredibly messy field that's still basically in its infancy. Developers want simple solutions to complex design problems, or at least they want some decent architectural guidelines so they can avoid constantly reinventing the wheel, badly. Remember when MVC was all the rage?

ECS is good though, it's a perfectly solid answer to a lot of thorny design questions. Where problems frequently arise is when you try to jam absolutely everything in your game into the ECS structure. In practice you're probably going to have a lot of data which lives outside the system and is not attached to any entity.