Totally different requirements. Some countries' fast food apps are for mobile ordering and delivery, some are a giant collection of coupons, and some are only for nutritional information. And some are just for promotional activities (only used for various promotional calendars.) After seeing all the different APKs I tried out a few for different countries out of curiosity and at least for Burger King they were entirely different applications with completely different use cases. To cram them all into one global app would be an enormous mess.

I think fast food chains are generally not actually owned by the same parent company. A company in India is licensing the Burger King branding and presumably some of the recipes from Burger King USA, rather than being a subsidiary thereof.

This is the real reason. For instance, McDonald's in India is itself run by two companies, one for North and West India, and another for South and East India.

Probably payment APIs, just like Uber. Or they are developed by different local app shops.