From the thread:
"T-Mobile has put in place some protections to prevent unauthorized transfers of your account to new SIM cards, I just had to deal with them last night - actually. Swapping SIM cards for a line must either be done in-store where your photo ID can be verified, or over the phone but only after confirmation of a OTP sent to account managers via SMS.
I know T-Mobile actually had some issues with this in the past, so even though I miss the convenience of going to t-mobile.com/sim to swap a card out I feel it's a much better solution security-wise."
Obviously it differs from provider to provider and time, but it'll start moving towards better security. 2FA has only recently gotten popular