More specifically: while one X chromosome usually gets deactivated in XX women, this inactivation ist complete. An XX Woman has one whole X chromosome and about 15% of the remaining X chromosome active for protein production.

X0 individuals will lack this genetic information and thus some proteins are produced in less quantity than usual. Which leads to incomplete sex development.

Thus since X0 individuals don‘t have the regular 15% extra X, they cannot properly develop gonads in the first place. Despite the later pathways being set to ‚female‘