The reason frameworks usually don't run in a worker thread is because it is difficult to efficiently cross the barrier between workers and the main thread. Either you need a shared array buffer or the contents of your messages have to be copied on write. Even web assembly has similar issues as it runs in the same thread as any JavaScript you have.

I do agree about making the distinction between sites and apps though. A lot of the contortion frameworks do is to avoid triggering layout excessively. You can't even get the position of a DOM element without layout rerunning to figure out where it should be. I suspect a lot of these quirks are legacy web behaviors more applicable to sites than applications.