That is absolutely an option, you can spec out multiple laptops with cellular radios. It isn't standard because it costs extra and most people aren't going to pay an additional bill for another cell line. Typically they're more common to enterprise environments with people out in the field a lot.

Hotspots are probably still the more common option though, at least in my experience.

Those used to be things in the early 2000's; cellular cards with sims that went into a PCI slot. Surely they still exist?

Pretty sure Linux supports eSims and some older laptops support Sim cards, you can add one to your PC as well. Personally I just use secure ways of communicating but it would def be useful when your traveling.

There are different cellular networks and frequencies in different places so computer manufacturers can't pick one module and expect it to work everywhere. If you're paying for service, you'll want to get the most out of it by having a modem that's up to date and maximally compatible with the network. So it makes sense to get a plugin device or a WiFi mobile hotspot from your carrier. A WiFi mobile hotspot can additionally be used with other devices.

Also mobile network hardware updates at a decent pace so it's nice to be able to update those single purpose devices at their own pace independent of your laptop upgrade.

And then carrier and FCC certification are an extra hassle for laptop makers.

Because it costs extra for little benefit. LTE was a choice for my notebook. I just set my phone to wifi tether, saved me $200.

Edit: or did you mean being able to make calls from your computer? I looked into this a while ago, since digitized call processing is just VOIP on a large scale. In my country at least, the providers only sell whole packages of numbers, usually to companies where you can rent one for use with your run-of-the-mill VOIP software. Of course it's nonetheless thightly regulated, because of regional and limited numbers and against abuse.

Because most places you'd use it already has free WiFi (home, office, train, coffee shop, etc), and everywhere else you can just tether from the phone, so there's no point paying a separate bill just for the laptop.

But you can still get a laptop with lte, it's an option if you need it. My x1 yoga has a sim slot. Or you can get a usb dongle.

They are. Skype, Signal, Telegram, whatever other million chat apps. The only thing stopping you using the big cell networks is the telcos stopping it to sell their access, and phones. And some of them may allow it for a fee, but I don't care enough to find out.