Patents and a quagmire of legal hurdles. They have long complained about Qualcomms pricing structure. A company like that has a lock on the industry even Apple can’t fully circumvent without significant costs.

It’s sort of the last piece in the puzzle. Once they get their modems, they become a fully unified system.

You talk about boosting network stability? That’s just the start when you control all the airwaves.

There was a prototype that popped up on ebay out of nowhere back around 2011. Seemingly made it pretty late into development before the idea was canned.

https://www.macrumors.com/2011/08/14/photos-of-a-prototype-macbook-pro-with-integrated-3g-cellular-data/

I’ve heard part of the issue is Qualcomm’s licensing is a percentage of the devices selling cost, so putting the same cellular chip in a MacBoom Pro costs a lot more than putting it in an iPhone. If Apple can make their own chips and doesn’t have to pay that fee it becomes a lot more affordable.

Maybe patents

Tethering burns through battery and a laptop's battery is much bigger. I believe Apple's smallest laptop battery has roughly three times the capacity as their biggest iPhone battery. Tethering works well for occasional use but if you've got a job where spend pretty much your whole work day roaming around to different locations, having it built in can be quite useful.

Unless physically disconnected it's a nice place for a backdoor