Why does my boyfriend still talk to his OLD girlfriends?

My boyfriend and I have a great relationship overall but he still talks to his old girlfriends (the ones he dated 2–3 years ago) and claims they’re still friends. He talks to them through Facebook or text but he never tells me about it and he’s even met up for coffee with them behind my back. He does it behind my back because he knows I’d go crazy and my self esteem is crap. I’ve broken up with him over this issue and we’ve always gotten back together because he says he only wants me. I do love him but I am ridiculously insecure and can’t handle thinking he’d rather be with these girls. Any ideas why he feels the need to talk to them? What do I do?