Writing a floating point literal requires A LOT more knowledge than writing a hex integer.
What is the bit length of floats on your specific Python compile? What happens if you specify more or less precision than actually available. Where is the underflow to subnormal numbers? What is the bit representation of information? Nan? -0 vs +0?
There are people who know this and need to know this. But float.fromhex() is already available to them. A literal is an attractive nuisance for people who almost-but-not-quite understand IEEE-854. I.e. those people who named neither Tim Peters nor Mike Cowlishaw.