Because they aren't interested in doing so, and know that there are no good ways to define these things.

What they want is the means to impose strict gender norms, and to persecute anyone who does not fit for any reason and in any way.

So, today they want workers who aren't super gender conforming to provide their birth certificates to use the restroom. Then they'll escalate until workers conform or get shoved out. After that, strict gender dress codes for all employees, then gender-specific roles... sometime down the line, a ban on married women working in state organisations, women unofficially barred from most workplaces & most roles, and ideally barred from being out of the house without a male relative as chaperone, blocked from having a bank account or owning property, and in due course... welp now they are property.