I don't know for certain but I certainly think most of those who returned home in 1945 had seen quite enough shit and violence to last them a lifetime. Equally, many might have enjoyed 90 minutes of screaming at a bag of leather - no matter who was kicking it - rather more than say, sitting at home, nursing memories. Also, because of their horrific shared experienceof war and loss, hostilities between rival fans simply couldn't have been the same.
I'm not for one minute taking away from your own lived experience. You saw what you saw and I believe you but...if my guess is right, it'd then be probable that some of that generation were still going to both grounds on alternate weekends during the 60s.
The thing us, the older generation can often be kind of invisible to youngsters. Equally, they probably had no interest in engaging with the generation that invented teen angst (not that that isn't a real thing but well, after what they'd seen in their formative years...).
Perhaps it's possible that they were all around you but - through no fault of your own other than your youth - they simply weren't on your radar?