I'm on the road constantly, and the ability to just use my laptop whenever/wherever I want without having to fuss with a hotspot, and to have graceful failover from wifi (like if I'm at a hotel on a Zoom call) would be worth quite a bit.
I know I would pay basically any price for this, and I can't be the only one.
And yet.
That's a hefty premium for a laptop, and is best approached with an external peripheral which multiple devices can share.
technically almost trivial to implement
Having to split each machine SKU into many to accommodate different regions + carrier locking isn't as "trivial" as you think
I had the same thought for Garmin devices, which have to be paired with the app on your phone to get online function (despite some features using wifi)
If you need the phone with a specific app, you might as well use Strava. At least it's compatible with any phone (Garmin devices can only do the bluetooth pairing with specific models)
Why don't these companies just give a sim card port, indeed.
I suspect it's a trade-off for Apple and they concluded that people who might need that can easily hotspot through their phone and thus wouldn't necessarily pay hundreds for a built-in cellular modem that would also require a second cellular plan.