>And I don’t get why replacing one is an all day job for three people.

Let's go through the whole process. You need a system changeout for a ducted central HVAC system. Someone comes over and gives a bid. You accept. Your HVAC person needs to go to a supply house and purchase all of the parts and bring them to your residence in a vehicle than can hold everything.

Assuming you have an existing system, the refrigerant has to be recovered with a special recovery pump. You can't just dump refrigerant to the atmosphere. This can take an hour or so.

You've got to find the right breaker(s) and then turn them off. Disconnect the power wiring. Then you've gotta disassemble the old equipment and remove it. Next you move in the new equipment. Rewire the power. Rewire the controls wiring. Assemble the new equipment, assemble plenums, cut holes in plenums, install collars, run duct work from plenums to boxes, attach and seal all ductwork with tape, mastic, etc. You need to strap up the ducts to properly. You still need to solder the line set at this point.

If your new equipment runs on a different type of refrigerant, in all likelihood, you need a new line set (the copper tubes that transport the refrigerant to the evaporator from the condenser and back). Why? Different refrigerants have different working pressures, may require higher flow, etc.

Once you're lineset is soldered back to a condenser AND the evaporator, then you must pull a vacuum on the system to A) remove non condensable gases and B) dehydrate the system so that your refrigerants are clean and the system is efficient. Properly pulling a vacuum is very time consuming. Most guys start by pulling an initial vacuum and then shutting in the system to do a leak test. You may find a leak, this takes more time. You need to repair the leak, then pull another vacuum. Once you past leak test, you pull a final vacuum. You don't want installers skimping on this step as this affects the overall lifespan of the system and system efficiency.

At this point, you can start the system (I've had new units dead from the factory... groan), then you can start charging the system with refrigerant. You need to let the system run for a bit to monitor pressures, monitor return air and supply air temps, etc.

Once the job is done, all of the old equipment and refrigerant has to be taken to specific places for proper disposal.

Everything above is a generally straight forward non-permitted replacement. Other installs may require new circuits run to the equipment, natural gas lines, etc. Some jobs require permits so now the HVAC person has to go to the city, apply for permits, submit plans, have the work inspected, etc.

There's quite a bit of work that has to be done which requires transportation, disposal, specialized knowledge, expensive tools, employees, and most important of all, insurance! Also, keep in mind that a lot of this work is done at temperatures FAR beyond ambient AND in low clearance areas, usually filled with itchy fiberglass. This work is also done with risk of bodily harm (electrocution, poisoning (hello phosgene gas), suffocation (asphyxiation from refrigerants), heat stroke, falls from height, exposure to industrial pollutants like asbestos, burns).