The bad part about trying to find outlets all the time is that not only are you spending an inordinate amount of time searching for a working outlet, but you are also giving up time to try and top up a battery; it's costing you your vacation.
This is just speculation on my part, but I would guess that the reason that (working) outlets aren't marked on a map is because Disney knows that the outlets would be overrun with people trying to recharge phones, tablets, medical devices and mobility devices. So instead of people enjoying the parks, you would see them clustered around outlets on light poles, next to bathrooms, etc. - and inevitably, there would be the "I need it worse than you" issue that would crop up, given the number of self-entitled Guests we all see.
There is also a potential liability issue for Disney - if a child were to stick a finger (or something else) in an outlet, if you (or any other Guest) were to plug in a device, and the device is shorted out and damaged or ruined, or if a Guest were to plug in a device that causes damage to Disney property.
I doubt we will ever see random outlets listed on a map; it would be more surprising to me if we don't increasingly see outlets switched off during operating hours (and only available to CMs after hours) and the creation of more "charging stations" (like MK/Storybook Circus or the MK/Rapunzel Bathrooms).