Why do guys in relationships follow other girls?

I know it‘s probably my fault but I get hurt when my boyfriend follows random girls on instagram and likes all their pictures. they‘re not models, just girls from our city or different cities. I told hin that it hurts me and he said he wouldn‘t do it anymore but I know that‘s not true. I don‘t wanna bring it up again because I know my insecurities are probably the real problem here and I don‘t want to risk our relationship and fight about it. He treats me very well and tells me I’m beautiful etc. Am I crazy for being bothered if he likes these girls pictures and follows them? What can I do?
Why do guys in relationships follow other girls?
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