Your UIs are likely still ass. Pre-made websites/designs were always a thing, in fact, it's (at least to me) common to just copy the design of another place as "inspiration". When you have 0 knowledge of design everything looks the greatest, it's something you kind of have to get a feel for.

Frontend engineers do more than just churning out code. Still have to do proper tests using Cypress/Playwright, deal with performance, a11y/accessibility, component tests, if any, deal with front end observability (more complex than backend, out of virtue of different clients and conditions the code is run on), deal with dependencies (in large places it's all in-house libraries or there's private repos to maintain), deal with CI/CD, etc, I'm probably missing more.

Twcs layoffs were due to AI cannibalizing their business model by reducing traffic to the site.

And what makes you think the backend is safe? As if churning out endpoints and services or whatever gospel by some thought leader would make it harder for an AI to do. The frontend has one core benefit, it's pretty varied, and it's an ever moving field, mostly due to changes in browsers, also due to the "JS culture". Code from 5 years ago is outdated, but Spring code from 5 years ago is still valid.

My time spent with Javascript applications has thus far been pretty brief (working on some aircraft cabin interfaces for a while), but a lot of the time ended up being on testing on numerous different types and sizes of devices, and making tiny tweaks to the CSS to account for as many devices as possible.

This has been a while; perhaps the latest frameworks account for all of that better than they used to. But at that time, I could absolutely see budgeting several days to do what seems like a few hours of work, because of all of the testing and revision.