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.
Coke is somewhat of an impulse purchase. At least it needs to be in your mind to buy it at the grocery store.
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.
Coke (and Pepsi) marketing is mainly concerned with affirming that drinking soda makes you sexy and keeping restaurants et al from changing their supplier.
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.