Look at how renewable energies are being subsided; a fixed prices system.

Small providers can deliver renewable energy electricity into one of the big four's nets and get a high, fixed price for it.

The big providers can't do this with their own net, as they would pay themselves. In the end, renewable energy electricity in their own area would have the actual price, and they love their old nuclear powerplants with all their thoroughly hidden and externalised costs sooo much.

Moreover, it doesn't make sense to go to another big provider's area and infuse renewable energy electricity there. They could take revenge and do the same in your area, and there you're effectively back at the original case of selling to yourself.

What they have is for PR and for having competence cores about the stuff.