Because you're not having enough interesting arguments now. Perhaps you have an instinct to want to grow and improve, but you're not able to easily access that in the here and now.

I sympathise. Sometimes it's not necessarily even about "winning" an argument, but just articulating your point well enough that things can move forwards. I find I am more likely to dwell on arguments where the other person was arguing against a point different than the one I was trying to make. In those cases, going over the argument in my head is usually me trying to think about how I could have better communicated my point in a manner that would allow the other person to engage with my actual points; perhaps then we could move forward productively and some change happens: either one of us causes the other to yield some ground, or I'm able to fortify my stance and come up with responses to new arguments. Either way, lack of closure sucks, especially when it feels stagnant.

Dwelling on past arguments isn't healthy, but I suspect you already know that, or you wouldn't have asked this question. Try not to beat yourself up about it too much though. Everyone ruminates sometimes, and trying to force yourself to not think of something is doomed to fail. It's more effective to find new things to focus on to help you to move forward, but that's an ongoing effort, and easier said than done.