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For buses, it’s quite common to have plastic foil with advertisements on them, but I’ve never seen that on an aircraft. Looking at how aggressively some airlines try to save money, they probably already came up with the idea to sell the space on the side of their aircraft for advertisement purposes – so why did they not do it? Is there any technical reason to not 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.

The time needed to apply and remove the advertisements would cost too much money. As long as the aircraft is on the ground, it cannot earn revenue. Better to forgo the small profits from ads for larger profits from operating the aircraft.

1) For a typical urban bus, that plastic foil has to stay on at maybe a max of around 60 mph. For a commercial jet, it'd have to stay on at 600 mph. And suppose it starts ripping off and gets tangled in the elevator...

2) You could repaint the plane at a cost of $50-200K How much does it cost to give an airliner a fresh coat of paint? but the ad would only be seen by the small number of people at the airports, so hardly cost-effective.