Part of the issue is that electricity is fungible. If I consume one watt-hour from my grid, I don’t get to decide where it comes from. The mix of generation is the same for everyone on that grid. Even if you segregate the grids in order to vaguely guarantee that you are only consuming green sources, you’re also making the “dirty” grid cheaper and thus easier for everyone else to use, and there are plenty of ways of capitalizing on that difference that nullifies the segregation. It’s a bit like arbitrage.