Why aren't more of the standard library functions const?

I'm working on a project which involves lots of functions with const parameters. There are lots of cases I've experienced where I just want to figure out the length of an array at compile time so I can call a function, and I can't because it requires calling a stdlib function to take a logarithm or something, where the function totally could be marked as const but isn't for some reason. Is there something I don't know enough about rust yet to understand, that prevents them from being const? Are const parameters considered bad practice?