That doesn't explain the first ~230 years of US history though, where police weren't this way and we had the same Constitution.

Assault rifles were banned for most of that time. Now I can go buy 10,000 rounds of green tip XM85 ammo and an AR platform at a gun show in an hour. I’m not saying that justifies militarization (that’s mostly war profiteers selling to police departments and right wing alignment with law enforcement) but OP isn’t completely wrong.

It’s possible it’s a bit of an “arms” race, the police are more aggro but so are the public. At least in public perception back before the 70s its was perceived that by and large there was “respect for authority” but that’s eroded over the decades for various reasons among them court cases asserting more rights for individuals where cops can’t just up and arrest willy nilly. But also movements like “sovereign citizen” leaks in places enough to affect behavior elsewhere.

Also weapons are relatively cheaper today than decades ago.