Because there are inevitably licensing and especially permit application costs, the major cost there being the time and aggravation required to get the permit from your local authority which is usually an intentionally dysfunctional Kafkaesque bureaucratic edifice to hell that makes the DMV look like the teacup ride at Disney World.

Fun fact: In my county you are required to pull a permit (with a three to six month typical application wait time) to “install” any heating or cooling equipment, which technically also includes plug-in space heaters and window mounted air conditioners. To rub salt in the wound, you also need to pull a new permit to “uninstall” said equipment, that is to say to unplug it and put it away for the season. Yes, the system is certainly working as designed! (You also need a permit to install or replace a water heater. I’m sure every homeowner with a water tank that just blew and is now standing in a basement 12" deep in water with every other resident of the house screaming at them that they can’t take a shower is absolutely thrilled at that prospect. I predict the compliance rate with both of these requirements is as close to zero as mathematics will allow.)