Would be interesting if we get these modems in laptops. I have no idea why Apple never put a modem in their laptops. They are portable computers after all, like an iPad. And iPads have a modem.
Cost, market segment size, and logistics.
Unlike iPads, only a subset of Macs would even make sense to support cellular.
Macs are limited to being used folded out with a keyboard and trackpad/mouse, so some truly unique iPad use cases (field usage for property surveying/inspections for instance) that require cellular connectivity would simply not be done the same with a Mac. You go someplace to sit down, and at that point you are on wifi or can turn on a hotspot (or just automatically connect via your phone).
The Mac is considered to be a more extensible device - it is more acceptable to have a separate device for connectivity and even to connect it via a USB cable. To compare, many people don't realize that you can use USB thumb drives on iPhones and iPads - they expect all extensibility to be wireless or through internet-based services. That makes connectivity an obviously required feature on iPhones, and a viable upsell on iPads.
It also is different than all other BTO options that Apple currently has for the Mac line - rather than just picking between a set of interchangeable panels, keyboards, SoCs and flash options to piece together, the computer would need to be designed for a separate set of antennas, which might mean a modified case design like they have with many iPads.
Finally, 5G hotspots are a broadly available technology and are cheaper than Apple could integrate a modem due to Qualcomm's fee structure. Businesses often get them effectively free because of the expected network usage fees, while laptops today are not something that a cellular network provider would subsidize. Businesses are not likely to spring for integrated 5G as a result, further reducing the market opportunity.
The option also needs to be at least marketable enough to justify extensive international network testing and qualification.
If they can reduce the cost to the point where it becomes viable to bundle a C-series modem into an iPad Pro without a separate "no cellular hardware" set of SKUs - and if that modem provides a more robust featureset - I'd say cellular as an option or default feature in Mac laptops also becomes much more likely.
I think for example offering cellular-by-default in a re-released "MacBook" line of ultraportables would be quite attractive, and they could use activation metrics to justify expansion to the rest of the notebook line.