There was an interview with a lead tech at Uber about this some years ago, with the conversation starting with "why is the app so bloated!?" (in terms of megabytes) and his answer also answers your question:

The smooth and simple interface of the Uber app is the tip of the iceberg. His example was that their users don't (and won't) reinstall the app because they travel overseas. If anything, the time when you've just stepped out of an airport is precisely when you want the app to work smoothly!

The hiccup is that many countries have their own payment systems, Byzantine tax codes where this may or may not be displayed up front to the user (in various currencies and formats), there may be local laws around taxi-like services, etc... Some of those laws apply to areas smaller than a city, or may apply only to airport pickups, or the CBD area during congestion, so on and so forth.

The "core" app might be a simple thing that you can bang out over a weekend with an AI and a decent UI framework, but then you need to "draw the rest of the owl". Don't forget that there must be a matching app for the drivers! Different categories of drivers offering services that may be local to a region and totally absent elsewhere: rikshaws, tuk-tuks, taxi boats in Venice, and who knows what else!

AirBnB is very similar to Uber in this respect. They have to deal with about a hundred countries worth of law, often down to the state level. There's fraud detection. Customer support. Integration with travel agencies. Government-mandated reporting. Etc, etc...