"[...] 90% of virtual assistants are initially programmed with a binary female gender. This matches the negative stereotype of women as compliant and available to serve."

Well...sure but it also matches the positive stereotype of women being nicer and more likable. Also maybe more trustworthy and reliable.

We can just make up reasons but those are just unfounded allegations.

Personally, I find female voices to sound nicer and more friendly. That would be my reason, as far as I'm aware.