There is no justification for that IMHO. The program text only needs to be in memory once. However, each process probably has its own instance of the JS engine, together with the website's heap data and the JIT-compiled code objects. That adds up.

I'd very much like a crash in one tab not to kill other tabs. And having per tab sandboxing would be more secure, no?

What do you mean? All these features are provided by process per tab.