Why is it so hard to believe in platonic friendship?

Tim and I have been best friends for 7 years. A few months ago, he started dating Margo, who is only 19 and doesn't have a lot of life experience (aka she's boring), but seems reasonably nice for the most part. Overall, I don't really have a problem with her except for one thing...she is so insecure that she is constantly whining to Tim about how she feels threatened by me, etc. And instead of telling her there's nothing going on (I have no interest in him and never have), he placates her immaturity by ditching me. We live 3 hours apart, and I used to drive out to visit him once a month or so, but I'm not allowed to stay in his house anymore (even if she comes over or if he goes to her dorm and I stay in the house alone) cause it makes her "uncomfortable." And when we do hang out, he spends the whole time texting her. The funny part is that he has cheated on her with an ex. And she has no problem with them still being in regular contact. But I'm an issue for some reason. What can I do?