The performance argument unlikely is applicable in this case. I suppose that an overhead of function call in Python make two operators faster than one function call.

Alternatives to fma() for exact computations are integer arithmetic (if all values can be scaled to integers), fractions and decimal numbers.

But since fma() is a part of C (C99), C++ (C++11) and POSIX (POSIX.1-2001) standards for long time, I don't have objections against including it in the math module.