It's done that way so the breaker box will fit between studs that are 18" on center, which is standard for USA residential construction.

You generally only see breakers on din rail in the USA in industrial equipment.

2 big reasons for that:

1. Fits better between studs as the other commenter stated
2. Easier and cleaner to route the 2 power phases. US plugs are famously ~120V but what many don't realize is that's a single phase of 120V, and there's two phases that go into the breaker box. By combining the +120VAC and -120VAC phases you get a full 240VAC for higher power appliances like stoves, dryers, heat pumps and electric vehicles.