[colechristensen]

The SIM, being a physical piece of hardware plugged into my phone, could easily be used as an attack vector for my phone company to root my phone. Hardware plugged into my phone is a much more vulnerable attack surface than control of network traffic.

[EvanAnderson]

The baseband is already at the beck-and-call of your telco provider and has much better access to the hardware than the SIM card.

[infinity0]

The point is your phone (if it were secure enough) could treat your SIM card like any other untrusted device accessory, and only let it do stuff it's allowed to do.

[network] <-> [phone] <-> [SIM card]

In theory. Not sure how well practise matches this though.