How do you know that? Desktop form factors are much more flexible, making them effectively smaller. They don't need a case - you can disassemble them, build them into cabinets and consoles; you can reduce their volume to a keyboard (or less).
They’re not looking for a science fair project, it’s a production piece of IT equipment.
Adding a bunch of bespoke equipment is an unnecessary risk when a well understood off the shelf product fits the bill. It’s just office equipment for doing basic computer tasks. A laptop is appropriate.
Much production IT equipment in industrial and many other settings is what I described. It's a mature, commonplace form factor. It's in cars (without keyboards), control rooms, server racks ...
That said, I don't know why they use laptops, and it doesn't look like you do either.
