It's because you don't do bulk calls. Sells, support, orders, etc. They all need office phones at a certain scale. Not that they couldn't do it with mobiles. But infrastructures for those kind of systems all assume a land line.
Decent deskphones are significantly more comfortable than cellphones, nevermind the better audio hardware.
You could make special apps with a UI made to help with the specific tasks, and with a good bluetooth headset make a nice mobile experience IMO. But yeah currently solid IRL keypads + phone handles are better than mobiles'.