That's because most contests (but not all) are limited to C, C++ and Java and they've practiced hundreds if not thousands of hours on solving those kinds of problems. Of course they will be more efficient in c++ than a person who is spending most of their time wondering how costly string operations are.

That doesn't mean that there aren't also people who have done the same and still prefer python (or some other language) not for execution, but development speed (and bignum, larger standard library...)