[antjanus]

Probably legacy reasons and the type of errors you can get.

Since JS used to be "sugar on top", it wouldn't make sense to completely eliminate the page when that piece of code which makes a logo flash doesn't work right.

Also, you can have JS errors coming from loads of places. What if an extension you use has a bug in it that triggers only on certain sites because of some stupid unicode issue? What if some ad has an issue like that?

And basically, it really boils down to: we all ship buggy fucking software. Everything has some kind of a threshold for errors (or errors that blow up only under certain conditions). It's good to have some built-in fault-tolerance that prevents an all-out disaster.