if this were popular you'd see old houses retrofitted with pluggable holes where you could feed the lineset through and seal as necessary for temporary heat/cooling, or perhaps even permanent.
the installation would be substantially easier if these were like 6000BTU instead of minimum 12000BTU per head, so it could be run on say, ~8 amps and be plugged inside.
The only problems I can see are:
-Size/Price ratio: The tonnage (9,000 BTU) is relatively low for its price--it can only condition a ~375 sq foot space. Though the lack of install costs probably makes up for this unless you're doing a DIY job with a Mr. Cool Ductless HP.
-Incentives: Utilities may not want to spend energy efficiency incentive dollars on these, since someone could easily move and take the heat pump outside of the utility territory; which is not a problem with permanent HVAC installations. Might be covered by the IRA's rebates for Heat Pumps though.
For cooling this exists in many forms: there are the window air conditioners, and "portable" window air conditioners which have a duct that connects to the window. The problem with the latter is that they're loud and the ducts result in a lot of heat loss.
For heating there are some portable air conditioners that use a heat pump but they're subject to the same issue described above. More common is resistive electrical heat in the form of coils or an oil filled container that radiates heat.
A portable mini split, where it connects through the window would give you a similar advantage as a portable air conditioner, but with more efficiency and being less loud.