JS engines spend a lot of their time in JITted code, which isn't helped by PGO compilation of C++. (I mean, the JIT itself is of course a kind of PGO compilation...) I believe in Firefox PGO is, or at least used to, be disabled because it didn't help much, but would occasionally cause crashes due to compiler bugs.
Running JIT code is only one aspect of the runtime. Parser, interpreter (if applicable), and GC benefit. C++ helper functions that get called from JIT code also benefit.