Why doesnt std make all possible functions const?

I have seen a message in \Option::take\\ that says: // FIXME replace \mem::replace\\ by \mem::take\\ when the latter is const ready, but when i look at \mem::take\\ (which is not const) it just calls \mem::replace\\ which is const. Also \Vec::len\\ isnt const when it just returns \self.len\\. Im sure there are more examples like that and I'm wondering why arent those functions const?