The telecom company has identifying information about you that hackers are unlikely to be able to fake - such as an home address to where they send your new sim card, or physical locations where it's much harder to work these kind of social engineering.

The telecom company also have more regulation and the stakes are higher for letting someone basically steal your number - it usually means they have a much stricter protocol to giving someone a new sim card - they'll require a physical presence, they can actually call your phone to verify you aren't a fake, they'll require confirmation of card ownership or an ID for states/countries that have them.