i have little idea on this & might be wrong, but i think its somehow regarding their government policy.. from what I read online, in mainland CN they need this government issued ID before you can register an online account like for gaming or social media.. since they made this app available outside their region & their ID requirement doesnt applies to all country, a mobile # is what they need
i remember registering a QQ account & wechat also, i gave them my mobile # but didn't get any verification code.. but after 3-5 months from my registration, i received a text of this code & then i gave up already since i received it too late and not interested anymore