Here in the UK thieves just use portable angle grinders that go through any lock like butter.

I have a hefty Abus “sold secure” (requirement of insurance) D lock that I lock through the frame, but I’ve watched enough lock picking lawyer to know this won’t stop thieves with angle grinders (most thieves now) or ones handy with a disc detainer pick.

In the US (at least for now) the angle grinder crews are mostly limited to nights when they can steal a lot of bikes a once and chuck them into a truck/van (ex: bike racks near student housing in college towns).

But the exception is always high-value bikes. That's why I called out bikes that aren't "obviously worth a grand+". If you leave out a nice eBike or a superlight road bike, the thieves have solid financial motivation to risk breaking out the cordless grinder and making a ruckus to steal it. Hell, sometimes thieves would straight up wait for people to come back to their bikes after brunch or whatever and mug them for it after they unlocked it in Albuquerque. No tools needed other than a knife.

What kind of society allows someone to use an incredibly noisy and obvious tool like an angle grinder in public, then ride off on a bike? Does everyone just walk by because they're afraid to get involved?

Yes. Are you willing to risk your life or serious injury over a stranger's property? Very few people are.

I've witnessed it, called the cops and took videos, but I'm not going to physically fight the guy with an angle grinder and little to lose - not even if it was my own bike. That was the same reaction everyone else around had.