Doing it that way they couldn't charge a lease to the homeowners. By leasing the batteries to the homeowners they're offsetting some of the cost through that lease, but also the selling point for the homeowner is they (presumably) now have a backup in case of a full outage

Also, you're reducing demand on the grid by having them at people's homes, not just reducing the demand on the power plants. If the batteries were centralized you'd still need to move that massive amount of power over distribution and transmission lines, which is actually just as big a problem as needing to spin up a peaker plant. In many areas the bottleneck is transmission not generation