The farm near me is at a main substation, I think it the only one in the area that was available with the land already purchased for expansion in mind.
They don't as far as I know have the room to expand it enough to actually provide power over a significantly longer period of time.
From memory it was one of the key reasons they chose the site, it needed expansion to cope with demand (new main incoming lines to cope with peak demand several days a month), and they had the space already purchased.
Spreading them out more requires the purchase of more land near existing infrastructure, for example if you put it at the local substations you're going to run into the issue that most local substations are built to service a very specific area with little room on site for expansion, around me most of the more local substations seem to be built with just enough room for maintenance work to be carried out, obviously with the thought that for expansion they'd just build a new substation where it was needed.
Building the batteries into non electrical company properties becomes very problematic in terms of maintenance access, and simply finding room for them in many instances.
If nothing else getting the various planning permissions and buying up the required land for a vast number of battery farms is going to be extremely expensive (I suspect they couldn't do it in many major cities so would have to build away from the demand), and time consuming.
I actually like the theory of the local battery storage, especially combined with renewable generating capacity, but I don't hold many illusions about the practicality if we were to be relying on such systems instead of a consistent baseline generator, at least with current and near future levels of battery technology.
