It is pretty peak HN to assume the world is only the US. Outside the US there are plenty of taxi apps. So, yes, we know from evidence that they are easy to build. The trick is getting customers and drivers for your app.
Do this exercise. Pick any country. See how many taxi apps available. They may not be called Uber and Lyft, but I bet there are only a handful at best, anywhere you pick (see Careem). That's the nature of the business.