In my experience, this happens in two ways. Yeah, sometimes a senior just overdoes it due to a lack of experience or shitty requirements or whatever.

But it also happens a lot that juniors just don't understand why the layer makes sense to introduce. For example, juniors will readily intermix IO and logic, because they don't yet understand that this makes code untestable and adds a load of additional complexity into already complex logic code. From that viewpoint, pulling all the IO code out will look like unnecessary complexity, when it's not.