Because they're irresponsible. There's no reason to hang on to this information once they've verified it.
That’s incorrect. They are obligated by law to retain it for 6-7 years for KYC purposes.
KYC for a gd phone number... honestly the whole attacks on E2EE make a lot more sense now with the background of that kinda shady stuff going on beforehand. praying for y'all, hope the digital rights situation gets better there.