"State management" is a fancy way to say "your Javascript stores data in variables in some way, which are updated as and when needed".

"Reactivity" means "when those variables change, some generic code already exists to update the DOM instantly". As opposed to writing specific pieces of code to make those changes directly.

Unfortunately, with React, "updating the DOM automatically" generally means: you update the data; React recreates the entire DOM as a new copy in memory; React runs a diff checker to find out what changed from the real DOM; React makes each individual change in the real DOM. With React (or other virtual-DOM-centered alternatives) being used for the smallest of web projects, so many small sites do all that work when the procedure ought to be as simple as: you update the data; you update the DOM directly.

It comes down to inexperienced devs not being able to consider which tool is best for the job at hand, and just hammering nails into every web project in the same way.