Girls come out as bisexual to please men?! That sounds like nonsense something that men might try to reassure themselves with! The bisexual girls and women I know are drawn to other girls and women for pretty much the opposite reason to this - because they don't like the shortcomings they see in men.
I also think more teenagers are trying to resist being pigeon-holed... Their sexuality is new to them; they are still finding out what and who they like... You could say that identifying as bisexual is the logical thing to do as a teenager, and it's a shame more teens don't feel safe to 'be bi'.
Unfortunately, being gay is much more stigmatised than being straight; and being a gay man/boy is much more stigmatised than being a lesbian. There are bisexual teens of both sexes, but you tend to hear about more bisexual girls because it is still (in most parts of the country, at least) difficult and dangerous for boys/men to come out as gay or bisexual.
As for self harm... Well, bisexual teenagers may be more likely to self-harm than straight ones, but they are less likely to self harm than gay and lesbian teens. Our society is homophobic. Lesbian, gay and bisexual teens are more likely to be bullied and discriminated against. And they are less likely to be supported when they are bullied and discriminated against. And bullying and discrimination makes people miserable and desperate, as we all know.
It seems to me that the implied question behind your post is the wrong one to ask. Rather than asking "Why would teenagers want to be bisexual, when that doesn't make them happy?" it would be better to ask "How can we better support bisexual teenagers to be happy, safe and free from discrimination?"
If it's knowledge you want rather than opinion, here are a couple of links that you might find useful.
NHS pages about LGB health, including mental health
This Google search excludes most of the chat/forum/opinion stuff and focuses more on research.