It's not just the seat width. It's a better seat.

But it's not even just the better seat. It's the cabin, it's the ambience, it's the fellow passengers - and it's the effect that all of it (including seat and catering) has on the travel experience. The extra you pay goes towards all of that.

In contrast, Economy Plus is literally just an economy seat in the economy cabin that has a few inches of extra legroom.

Obviously, if you are impervious to all of the benefits, then you will value PE differently and you may think it not worth the money. In that case, sitting in an economy seat in the economy cabin may be the better choice for you. However, the airlines' experience is that enough passengers do value the PE package to pay extra for it - hence the premium for PE seems to have been universally creeping up over the years.