I've answered it twice. Its easier to use the thing you already have than it is to make something new. Is that really difficult to understand?
> you could've registered a brand new account with username, password, email confirmation, and OTP 2FA, on any service that supports those
And then spend the many, many, many hours to share that new unique identifier with all the people I want to talk to. Or I can just continue using the same identifier they already know me by and have known me by for decades.
> I guess you've never seen bad actors buy fake ID/SIM accounts as quickly as you can block 'em
Whaaaa I thought you just told me a phone number is such an incredibly hard and challenging thing to get, now you're telling me anyone can easily get them anytime they want?
So phone numbers are incredibly challenging to get and yet people get them all the time easily. Otherwise, if phone numbers aren't hard to get and anyone can just freely get them what are we even really debating about?
For billions of users, having a phone number as their identifier isn't a challenge and is for sure the easier process and having to make a new unique identifier is a bigger deal and introduces far more roadblocks to effectual adoption. Its why WhatsApp uses it, its why iMessage uses it, its why Telegram uses it, its why WeChat uses it, and many others.