What stops a 90%-pure language to dectect pure functions and do the exact same optimizations? I'm not saying that is decidable in all cases (can't come up with

What stops a 90%-pure language to dectect pure functions and do the exact same optimizations? I'm not saying that is decidable in all cases (can't come up with a meaningful counter-example either), but usually a 90% good enough solution is better than a 100% solution that makes you go out of your way to do very simple things (like print to stdout).