Two reasons occur to me:
1. Situations off the grid or with unreliable power (eg blackouts) 2. Transmission costs are high, so it's better to have the storage close to the source.
Even in the second case, it seems like the power company could distribute these. I wonder if they are just more regulated/slow moving/immune to marketing.
My understanding is a primary limitation for these batteries is cooling. Cooling ability is relative to the surface area of the battery. One giant battery has much lower surface area than a bunch of small ones.
Also having a large number of batteries lets you put different cells on different recharge cycles to maximize battery life. You can use a small number of worn out cells for brief minute-to-minute fluctuations while you keep fresher cells on longer cycle that optimizes lifetime. With one big battery you're just stuck with whatever wear characteristics the usage curve gives you.
Also availability of lithium cells isn't that great at the moment. It'll improve as factories come online.
Besides, I guess there should be an added advantage to monitoring and using (charge/discharge) the batteries better than letting the user do the maintenance like preventing discharging to zero, temperature etc.