Exactly, no one is saying you can't see your neighbor if you don't have errands to run.
But the fact is you won't be doing that anywhere near as much if the way of doing the errands works against seeing people as if it works for. Defaults matter, a lot.
I agree that spontaneous interactions are important, but we get spontaneous interactions via density, whether it's tiny but compact towns (walled villages, even) or large cities.
Busywork and drugery should not be the foundation of social interaction. Period.
But the fact is you won't be doing that anywhere near as much if the way of doing the errands works against seeing people as if it works for. Defaults matter, a lot.