Like many things in the US, there's no centralized authority that mandates this sort of thing. Some states have laws around this some don't. For those that don't, some counties or cities might have laws around this. Belgium of course has a stronger central government, small land area, and a small population, so I'm not surprised that something like that would be done country-wide.

The shelter in my city chips every animal before anyone can adopt them. It's honestly bonkers to me why anyone who has a pet wouldn't chip them. It's cheap (especially when considering the cost of a regular vet visit), and can save you from lots of heartbreak later on.