Why don’t airliners have temporary liveries?
They do have temporary liveries. This is very common in wet lease operators, where an aircraft is flown for an airline during a peak period or when their planes are down for maintenance. Airlines want customers to feel they are getting the brand-name product they are paying for; they don't want customers to board a generic white jet.

An airplane will be painted and decals applied only for several months. Just like motor vehicles, 3M makes aircraft film wraps, including models specifically for short-term use.

It is also common to have a livery for a special event, say the Olympics. You can expect ANA's jet for the 2020 Tokyo Olympics to be returned to normal shortly after the Olympics are over. You can see Lufthansa and Egyptair dressed their planes up for the World Cup. Egyptair's photographic livery can only be done by printing onto a decal.

This video shows Alaska Airlines paint essentially an ad for Disneyland and the Cars movie franchise. They paint the simpler parts like the clouds and the large solid areas, but at 1:28 you can see them apply decals of the characters' faces. They say it took 29 days to paint. This is another video where they're simply applying a decal to advertise another movie.

So, no, there is no technical reason not to have a temporary livery and such schemes are done regularly. It's merely an issue of marketing and customer impressions.