Because a solar home (without batteries) is producing excessive power during parts of the day, relying on the grid to accept it, while using power from the grid the rest of the day. While those times may balance, you’re still relying on that power grid for both. That’s hugely expensive infrastructure that you’re no longer paying for. If something happens to your solar, your home can still use grid power: that’s hugely expensive infrastructure for “just in case” that you’re no longer paying for