Everyone (in Joel Spolsky meaning of everyone) already had an gmail account. Not giving out phone number was a feature.
When Whatsapp started out, it was free only for the first year; after that, they wanted a payment. What this experiment has shown, that people did very short term decision without regards for long term.
That's probably because you lived in a tech bubble already 10 years ago.
I reluctantly installed WhatsApp only after everybody around me started using it, or (the more tech savvy among them who already knew how to "open the internet"), switched away from Facebook to WhatsApp.
Everybody who had a phone also naturally had a phone number, and the phone numbers of their friends were already stored on the phone. Nobody outside the "nerd sphere" had a Gmail account 10 years ago, especially not outside the US.
PS: I don't remember ever paying money to WhatsApp, I think that was a short-lived experiment that didn't work out right before the Facebook acquisition.
When Whatsapp started out, it was free only for the first year; after that, they wanted a payment. What this experiment has shown, that people did very short term decision without regards for long term.