I don’t think that many iPads are sold by cellular carriers - and I’m sure many carriers would be happy to have phone functionality: there’s good money to be made from charged calls. Or people that pay for calling - but rarely use it, due to just being a secondary device to make calls. Last but not least, Apple could selectively restrict capabilities through carriers profiles.
I think it’s rather Apple that wants to sell iPhones to tablet users.
The iPhone is intended as the centrepiece of the ecosystem.
Same reason why Apple watches do not work with iPads.