Because if I'm Safeway and I can get elderly people to come to my business (for shopping, prescriptions, etc.) they won't be shopping somewhere else. The business gains/retains the clientele by providing the driving service.
Ok, so it’s an interesting idea to use personal transit, but it could be done already without self driving vehicles, it might be economical. But it would be pretty niche.
Honestly, I'm surprised it's not something that Google have already built heavily into Maps. People often search for what they want (e.g. burger, pizza) - this would be an obvious time to offer deals or discounts to drive customers to particular businesses. You then expand to compete with particular retailers - someone searches for Nike specifically, they get offers from the nearby Adidas store.
This is not really any different to what happens with Google searches, where advertisers can buy keywords related to their competitors.