It's a clever way to disguise their unit economics to look better.

Their marketing budget is mostly going to pay drivers.

Put another way - you could be saying - why not pay drivers EVEN less? Well, they're paying them the least they can already. You can be sure of that.

If they did actually paid the drivers more and not disguise it as marketing - then their unit economics wouldn't look good - and when the business as a whole doesn't look good either - that's not a good look.

They only need some occasional reminders that might be targeted at the few people that haven't used or heard of friends or family using an Uber before.

Conversely, I'm not going to take an Uber ride tomorrow because they showed me an ad today. When I need to get somewhere I'll look at my options and choose the best. So long as Uber meets the minimum level of me being aware of it then theyre good.

Marketing is as much about keeping your customers from going to rival providers as it is about finding new customers. If you don't want people to leave you need to remind them why you're better than the exciting new company that's spending VC money to take your market share.