The thing about some of these people is I think some of them truly do not care, they just want to seem like a good person by saying they'll be there for you.
They don't actually think about what "being there for you" really means. They're not willing to sacrifice their time to help someone, they're selfish.
Then I think there are others who may care and say they do but they have their own issues they need to work on. They feel they're not qualified to help because they don't even have their own lives in order. These types really should not tell you they care, they should just remain as acquaintances and not friends you rely on for venting serious problems.
Anyway, I've dealt with this and it does suck.
I tried to kill myself a while back, let a friend know (which was really hard to do) and she said she'd be there for me.
She then ignored my texts and attempts at hanging out for a few months. I was at a really low point and it was like being kicked while I was down. It felt like she wouldn't care even if I did kill myself, and it made me think about ending my life even more.