Well, one aspect could be that they do not want to pay Qualcomm a licence fee for every MacBook sold. Perhaps once Apple has finalised their own modem it will make an appearance.
But on the flip side of that, do we really need a modem built-in?
The vast majority of places we go to these days will have free WiFi for us to use, so there’s no need there.
For when we’re out with WiFi areas, well, who owns a MacBook and not a smartphone they can use as a hotspot? I’d wager that number is minuscule. So why pay for yet another cellular contract, when we have a perfectly good one already in our pocket?
Personally, and it is just personally, I wouldn’t dare speak for an entire market. But I have unlimited data with the contract I use with my iPhone. For years I’ve used that as a modem/hotspot when we go on holiday and there’s no WiFi, along with a Kingston MobileLite travel router, to share that data with my family.
Even when we’re in the middle of nowhere - when my wife drags me kicking and screaming away from civilisation, to stay at our caravan, there’s still fast 4G (and eventually 5G). Even with more than one of us streaming video, it’s never an issue.
So, there’s a market for it, just as there must be for people who have cellular iPads (I do, though I’ve not used it’s cellular in years, I realised paying for more than one was folly), but I think it’s probably a pretty small market overall.