Why does it seem as though my Straight/Hetero friends are ignorant about HIV/STDs and safer sex?

I'm bisexual and I've noticed (as have my other GLBT friends) that our heterosexual/straight friends DO NOT get tested for HIV or other STDs and for the most part do not practice safer sex or get tested with their new partner(s) before deciding not to use condoms or have unsafe sex, why is that? Has anyone else noticed this with their heterosexual/straight friends? I'm not saying that ALL my hetero/straight friends are like this but many are.