They have to. Feeding your own home needs some setup but is fine. But electricity companies require you to disconnect generating capacity from the grid when the grid is down to make it easier to effect repairs.
But that's more a policy decision than a technical restriction. We could change it so power can flow on both sides of a fault instead of only the "upstream" grid side.
With battery systems getting so cheap maybe community batteries will become a thing where a neighborhood exports it's solar too and is it's own small grid.
a) government mandates that turn over existing grid infrastructure to such a project, because the existing grid infrastructure is almost all privately owned
OR
b) building new infrastructure to create an isolatable local grid
I mean it isn't though: it's defense in depth - policy is you must disconnect. Line workers will drive a ground stake in on both sides anyway, but if you don't disconnect then they'll just short your inverter to ground.
But that's more a policy decision than a technical restriction. We could change it so power can flow on both sides of a fault instead of only the "upstream" grid side.