Last mile electric transmission is mostly a natural monopoly. Very few customers are willing to pay for two separate connections, and routing overlapping electrical transmission networks is expensive in space and likely unsightly.
There's a few options for natural monopolies. Regulated standards and practices, and tarrifs can be effective. Public ownership can be effective. Mandating divestiture can be effective.
A market works pretty ok for generation. Although, a system operator that coordinates to provide stability seems like a requirement.
I think there's potential for a market approach in long distance transmission; the operator could identify unmet transmission needs (sector A often has excess/curtailed generation, sector B often has excess/curtailed loads; maybe we should increase transmission capacity through those links), and run a bid process to solve those needs. I think the high price to build long distance transmission means you do want to have coordinated planning though: you don't want to massively overbuild transmission capacity on one path.