One reason has strictly to do with marketing. The manufacturers have to protect their network of installers and factory service contractors. Therefore, they do not like to allow the SALES of equipment to be taken away from them. A small contractor cannot stay in business doing an occasional "ductwork" job for folks like you. They need to be able to make a profit on the markup of equipment sales.
A second reason is technical. The proper sizing of HVAC is not something that can be pulled out of the hat. Nobody wants to end up responsible selling you a system which ends up over/under and doesn't work.
Finally, the installation of HVAC requires a skilled technician ( a hands on guy, not an engineer. ) AND if air conditioning is involved, the technician must also have an EPA license.
I am not making excuses for anybody. I am telling you the way it is.