Updating and checking over and over is usually more about getting the ux right than making it work or fixing bugs. A tight feedback loop makes a huge difference when you're iterating on look and feel. If you have to wait 10 seconds and fill out a form or two to restore the full app state after each change, it becomes a nightmare and you'll likely just give up on making a good ux.

There are also lots of frontend bugs that aren't really bugs in the compiler sense, but just cause the user to see a slightly wrong or confusing thing for a given state. Tests can help, but for a UI with a lot going on, there are usually too many possible paths to write tests for them all, so doing a lot of QA as you go is hard to avoid.

Typescript offers a pretty good balance here since you can turn off typechecking for live reloading and instead check types in the background or as part of the test suite.