Agreed, not an equivalent system.

Another scenario where this causes issues that a login equivalent can improve: imagine losing your phone including SIM card. This then causes a full interruption of all services tied to that phone number including actual phone service, and SMS, until the SIM is replaced. This usually takes hours/days depending on your location or carrier.

The other big difference in just swapping out sim cards is the need for a full phone reset and the interruption to the service of whomever you're borrowing the phone!