It's not about raw performance. It's about getting shit done and keeping shit highly available. In large systems, shared data isn't mentally optimal to work with (see: every multithreaded program with shared data segfaulting for some unknown reason this very second).

We can always drop down to C or asm to make ultra-shared data structures with minimal overhead. We could always do that. But people end up wanting to spend their precious lives making a difference in the world and not checking the return value of malloc thirty times per day.