Phone calls and SMS messaging are part of a Cellular/Mobile Calling Plan, which costs much more than a cellular data-only plan. (Anyone remember when text messages cost $0.25 each?)
iPads are pretty awkward to use as phones (even minis), assuming people want to hold the phone to their ears rather than use them only on speakerphone or with a headset. Basically, use a headset or kiss privacy goodbye.
So should the telcos and Apple get together and decide that they could offer SMS + cellular data capability? Sure, why not? Kinda like the old Blackberry days. I'm sure they'd charge more than $10/month.
"Which plan do you want, cellular data-only, cellular + SMS, or full mobile calling with text and data? Unlimited calling, text, and data?"
"Why are you asking me all these questions? All I want is an internet connection for my iPad when I'm at a construction site!"
Effectively, an iPad with cellular data is similar to using a dedicated cellular hotspot with a laptop - those are also data-only devices.
Whatever. I have all my needs addressed without paying extra for a cellular iPad. When I need to, I connect my iPad to the cellular hotspot on my iPhone. I have Text Message Forwarding enabled, so all my SMS messages are sent to my iPad and Mac as well. I'm also using Calls on Other Devices, so when my iPhone rings the call also reaches my iPad and Mac.
The principal reason people seem to want to have that iPad "phone number" be active for SMS and/or calls is to have a second phone line - home and business, let's say. It seems a less convincing argument now that iPhones have dual SIM capability, but I'm sure there are some people (a fairly small percentage of cellular iPad users, I'd wager) who can make a logical argument for it in their particular situation.
I also suspect for some people it's just the principal of the thing - the phone company assigned a number, and that precious number seems to be going to waste. Again, a fully-capable telephone pays the monthly fee for a fully-capable telephone. A data-only device pays the data-only rate. No free rides.