If all you’re doing is submitting a form with Ajax, sure, jQuery would be fine. But I’ll venture that most frontend apps are at least somewhat more complicated than that.

My side project is a web app that lets you create music visualizer videos. There’s a ton of UI, state, complex interaction between parts, etc. If I weren’t using React, I’d be using something — but it would surely be another full-on framework, not a simple library like jQuery.