Why would someone catch their partner cheating and decide to not confront them about it and instead, just end the relationship?

Background information:

Man and woman date for 5 years. Towards the end of the 5 years, the man finds out the woman has been cheating because he found medication for an STD in her room. Instead of confronting her about her cheating and freaking out on her, he pretends he never saw it and just breaks up with her without giving her a reason. Why would he do this? I feel like most people would freak out and confront the cheater.
Why would someone catch their partner cheating and decide to not confront them about it and instead, just end the relationship?
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