No accounting for taste, I suppose. Writing JSX is a nightmare to me. It's like we've forgotten 20 years worth of hard-earned knowledge about the value of separation of concerns.
What is the value of separation of concerns when it comes to UI? Because I find not colocating the logic and styling for a component to be very annoying when it comes to looking for the right files.
The component DIR should have css, logic/js and template/markup. Seperation of concerns right down to component level. It really comes down to how organised the devs and their tools are.
Do a 5 why for separation of concern and you'll find JSX is more logical.
You want things edited together to be closer in code also. A single file or couple of files next to each other that decide structure, behaviour and view of a component is better than few CSS/JS for whole site.
I agree with you about separation of concerns, but I don't think JSX is the guilty party there, so much as the overarching frontend ethos that pushes the idea of "single file components" to extremes.