For the same reason your Ethernet card and wifi chip have MAC addresses, except the cell network is a single giant address space spanning literally the entire planet. Even better, it's paid-access you wouldn't gain anything by periodically randomizing it unless you also anonymized payment. On top of all that, it's all the way down at the hardware level, and I bet that cell network equipment is sensitive enough to tell what model of phone you're using by looking at the features its silicon supports and then deanonymize you based on deviations from that model introduced by your phone case and which pocket you carry it in and how you hold it.

Is this where I admit that I change my MACs whenever I can to something random when I find the option in my device config?

Yep. Although, as I pointed out, IMEI randomization doesn't do anything if your payment and use authorization isn't also anonymous.

Probably for the same reason many technical horrors arise: It seemed reasonable at the time, it was the simplest solution, somebody added it to the spec for some reason.

Then things like this get used in such a way.