If there's a 3rd party aggregator (for taxis, restaurants or whatever), the final price that the customer pays would include both the aggregator's profits + the individual service provider's profits.
If the individual service providers all came together and made an app themselves, the aggregator-profits can be eliminated, resulting in potentially reduced prices for the customers.
I think (probably wrongly) one of the reasons they don't usually come together is because by making such an app they would effectively be helping their competitor(s), which they may not necessarily want to (because they don't see the bigger picture?).
In my city (in Asia) a subset of taxi drivers realized this and even made an app, but it didn't really work well due to poor marketing and because visitors to the city didn't want to set-up a new app just for 2 days etc.
On the other hand, if all the service-providers do co-operatively make an app for their services, what prevents a cartel from developing? The government?
I guess there should be a high baseline level of trust to make it work.