en-route ATC units do not get VFR flight plans.
Sounds like a technology issue then. If someone wrote something that worked then it will cut down a whole load of radio
if people wanted to file.
Otherwise you'll have to pay more controllers and open more positions to keep the airwaves clean.
it usually isn't a legal requirement
Same over here too. It is optional and so it should be, but it's good to file if going to remote areas for emergency sake.
If we use flight following, handovers are mostly automated and electronic nowadays.