Bixby, Siri, Cortana and "OK Google" are women's names? The only one with a woman's name that I can think of is Alexa. Granted they all have a woman's voice, but doesnt seem like that was your point.
Siri is absolutely a fairly common Nordic woman's name, I know several people with that name.
And Cortana was named after a female video game character.
They voices are male in some other countries. In the UK, for example, Siri is voiced by Jon Briggs.
in fairness, "Cortana" and "Siri" both sound pretty effeminate to me (and speakers of romance languages)
A vast majority of personal assistants are female, that could be part of the reason