Yes, there are lots of those, because the travel industry realized people want to compare offerings before they make their priciest purchase of the year. Customers would themselves do manually what all those aggregator sites do automatically and they care less about who they are dealing with and more about the price tag.
In comparison, a taxi ride might not even be the priciest purchase of the day.
The service will automatically hail from whichever service is the cheapest. Then pull an Amazon and expand into only the most profitable markets.