You and your nearest 400 neighbours plug into a non-blocking 1g port network. Find you can do 1g to your neighbours

But you need a 400g uplink (and remember to account for resilience), so you go into a switch with 40 400g ports, ok, that's 1.6tbit

Scale this up to a small town with 20,000 people and you need 20tbit of backbone to serve your residents with 1G non-blocking, very few will ever use the full 1G

Sharing bandwidth makes perfect sense. If you want uncontended (to where? Local IXP, a major IXP? One on the other side of the world?) then get a business line which gives you uncontended connectivity.