I saw that question on Tumblr today. I can't answer for every bi woman, but here's my answer as it pertains to me:
For the same reason I don’t try to date anyone else of any walk of life...
I’m not in a good position in life to date anyone.
I don’t want to bother anyone with my ADHD.
I have too much mental baggage from growing up in an abusive household.
I’m not attractive and can never hope to be.
I don’t like myself and I don’t like I’m good enough for anyone.
I’d rather be friends and sit around sharing hobbies.
I don’t trust easily and being gender critical while a good chunk of the western population is gung-ho trans ideology, makes it even harder.
I also like my space and privacy mostly. It would be too much trouble to let someone into my heart. One time I did--even over the internet--and it had disastrous results for me.
When I say "disastrous", I mean end-of-the-rope and praying to be hit by a train because I felt so worthless, disgusting, used, non-human, extremely insecure, etc. and it wasn't pretty. It me a few years to heal up, but I still have the scars and the memories that haunt me.
And that's what I would put on my Tumblr if I had my original name.