Because it’s not just a convection oven that’s needed, it’s also venting the moisture that comes off the product being cooked. It also needs to be powerful enough to keep the temperature up while that moist air gets exchanged. That’s a lot easier with a small cavity like an air-fryer than a regular sized oven. Commercial combi-ovens can do the same thing, and maybe there’s some high end consumer models available, but it’s a lot easier on the pocketbook to get an air fryer than a full size oven. I’m not totally sure, but they might also use impingement(concentrated streams of hot air, as opposed to just regular convection), which is another thing that’s available commercially, usually in pizza ovens or things like the merry-chef/turbo-chef(Subway’s sandwich toasters) but not common in consumer equipment.