Because dynamic typing is terrible for big projects, and typescript's fake sense of security doesn't cut it. When you know that your code works you don't need to check it in your browser every nanosecond (although you could).
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.