It is because we need to have to create your social network. Phone numbers suit very well for this.
Discussion
There are great ways to build social networks without phone numbers (look at simplex and Nostr for example), but iff you insist on using phone numbers, stop advertising as a privacy product.
More importantly, why isn't your code FOSS? How do we know your app isn't full of telemetry and other privacy/security violations? In fact... when a company refuses to FOSS, I must assume the software is full of shady shit they don't want me to see.
Look at our GitHub. Everything is opensource.