Yes, a long list of the backward US safety regulations are shameful. Also yes, there are shamefully backward lighting regulations in Europe/rest-of-world, too. It sucks, but it is what it is, and it's not likely to change very much or very soon.
The trap here is assuming that any/all flashing brake lights are necessarily a safety improvement. The U.N. (Europe + rest-of-world) Emergency Stop Signal is a safety improvement as specified, but just flashing the brake lights every time they go on, or flashing the CHMSL every time it goes on, or any of the rest of the random setups, really aren't. Furthermore, the American situation is not the same as the rest-of-world situation: there are red rear turn signals and hazard lights, so in that context flashing brake lights (even ESS-type setups) would be adding yet another dimension for drivers to have to figure out on the fly: is that supposed to be a dim red light or a bright red light? I guess it's bright...oh, look there, yeah, it must be bright because the CHMSL is lit, too. Is it a flashing red light or a steady-lit red light? Oh, it's flashing. Is it a red light on both sides of the car, or just one? I can't tell...well, is it a fast-flashing red light or a slow-flashing red light? It looks fast, now am I seeing a fast-flashing turn signal because there's a fault in the system, or is that urgent braking? Or wait, is the driver pumping the brakes or riding the brake pedal? Meanwhile the crash happened before most of these questions could be asked, let alone answered.