why do girls give bad dating advice

Well there's certainly a spectrum. Some definitely give better advice than others. Off the top of my head I'd say Marni Wing Girl is probably the best female dating coach. I've heard her say things and describe things much more candidly and honestly than really any of the others. Apparently she's good for mainstream too. 

Anyway there's two main female roles at play in your question, and each can introduce their own distinct dysfuction. 

1) Female dating coaches- As far as why female dating coaches usually mostly suck, from what I've noticed they just don't have the energy and force of will to enact the changes they need to in their students. They're too "polite", too "cognizant" of their emotional concerns. This ends up being detrimental. Sympathy comes after, skills and training and understanding come first. Quite a few of my students required a decently strong "pattern interrupt" to get them off emotional wavelengths and mental extrapolations that were so engrained in their psyche they'd.

2) Regular women - Women are more looking for a partner in a man than just physical intimacy. Men can certainly want this too, but typically they're much more independent and autonomous in their pursuits. The problem for women is that the things that make a guy "good" at relationships, don't necessarily equate to good life skills. Think Jared Leto in Requiem For A Dream. So on one hand women expect their men to be super cordial and endearing to them and demonstrate an emotional wavelength that's suitable for long-term, but on the other hand this ends up dragging in a bunch of momma's boys. However sifting through these suitors is definitely easier and less dangerous than sifting through a bunch of "assertive" males. These guys are certainly alluring to women, but they also can be much more punishing. Not that plenty of "nice guys" aren't also emotionally stunted weirdos, but they're typically more beholden to a woman's feelings.