If they wouldn't, you'd find dead motorcyclists every 50 meters
You know, as car driver, that you have constant chances that motorcycles (especially scooters) are passing you on both sides.
If car drivers would drive like here, you would literally have downed motorcycles every block.
Italian (and most European) drivers tend to be more
aware of vehicles around them because nearly everyone on bikes/scooters/cycles two & four legs filters around stop-and-go traffic.
You don't really need to check your mirrors, you just
know someone's there thanks to engine noise, honking... Part of it is because most people ride two wheels
before they get on all four, so nearly everyone knows the drill.
I think the main reason for (relatively) low accidents is that everyone is going at low speeds in stop-and-go traffic so:
1. you're going so slow that all you need to do is put a foot down, and
2. it's so packed, there's really nowhere to fall
Most of the deadly accidents happen when/where people drive fast: outside of the cities and/or at nighttime, when there's little traffic and people want to get it out of their systems.