You’re thinking about it from a consumer point of view. From the carrier point of view, they don’t want it to be easy for you to decide to switch to another carrier (They want their service to have a high switching cost). So they have various roadblocks like this, to make it a bit more frustrating to switch carriers. Of course they can’t make it too difficult where they would face legal pushback, so there’s a lot of stuff like this that feels random or suboptimal from a technical point of view but helps their bottom line in aggregate.

Remember back when text messages or phone calls were free as long as you messaged/called someone on the same carrier? That wasn’t about passing on savings of actual interchange fees, it was about incentivizing you to be loyal to your carrier and get your friends on board as well.