Are you from a first world country? It sounds like you're from a first world country! There's a legitimate reason to use phone numbers - email hasn't worked in many third world countries such as India. But everyone understands and has a mobile phone already, and thus it becomes perfectly manageable. This is why whatsapp succeeded, they zeroed in on the phone number (and the OTP instead of an account) as a way to authenticate and identify users. If someone's in your contact list, you instantly can connect with them! The level of integration of whatsapp into every aspect of the average indians life is insane and incomparable to what you might see with email or any tech software idea in a place like the US.

Furthermore, almost no service in India uses passwords, everyone uses OTPs. And initially people were not aware and there were lots of social engineering fraud, but now everyone knows not to share OTPs with others, and frankly this looks like a far more secure method than the password based auth systems prevalent in the western world.

Further there's an even bigger incentive in places like India - easy control of spam. I am not aware of any easy method to procure phone numbers here without linking it to your identity, so it becomes much harder for spam account creation. You could call this draconian but I don't see any problem with this behavior from the company's perspective.