Both of these businesses have what are called supply-side network effects (https://en.wikipedia.org/wiki/Network_effect), which are not as strong as demand-side effects, but can still lead to dominant market positions. In more concrete terms this means that for you as a consumer, there is no DIRECT additional value of someone else joining the service (you don't care if your friend joins Uber), but the value of the service does go up with every additional supplier that joins the network (for Uber, this would mean shorter wait times when you want a car, etc.) The defensibility of these types of services is created through their supplier network, and their ability to create more value for those suppliers (and consequently consumers as well) that the competition. Uber / AirBnB can then use the revenue from their leading market position to innovate faster than their competition, create more services for consumers, pay for distribution / new customer acquisition, spend money on marketing and awareness, etc. all of which leads to them creating a massive moat around their market that makes it nearly impossible for anyone to compete with them.