Howcome? Same banks of 18650-batteries and inverters are cheap. Only problem are maybe the safety issues. But in multistory houses the batteries would be in the basement and only those display units are in the homes.
Because large banks can amortize usage over many homes. A battery bank for your house needs to be large enough for your worst case usage even though you rarely get there. If you add up all the breakers in my box it is far more power than the power company can supply to my house - but I never use them all at once. My house has a backup generator that can supply far less than the power company: it is enough for typical use but I have to be careful when I'm using it.
A large bank can take advantage that typical is far less than worst case, and not everyone will hit the worst case at the same time. Thus a large bank to supply the same number of users can be far smaller than every user having their own small bank. (though it is possible for all the small banks to supply to each other)
