Utilities get a local monopoly and guaranteed tariffs in exchange for the considerable investment in building out the supply grid and generating capacity, and the obligation to maintain it.

If individuals are allowed to opt-out, that changes the financial promises made to the utilities. Of course this was mostly done at a time before it was economically feasible for anyone to go off-grid with solar and batteries.

I quite honestly prefer this arrangement. I have zero desire to own and be responsible for the maintenance and safety of tens of thousands of dollars worth of on-premises solar/battery/electrical transfer switch gear. I'm quite happy to pay the local utility to run a cable to my electrical panel and have them be responsible for everything outside the walls of my house.