Maybe AMBULANS is seen written more often nowadays than POGOTOWIE. I've never seen KARETKA POGOTOWIE on the sides of an ambulance.
Because ambulans sounds more modern - more wordly and dignified. Karetka is an old word which may be associated to an old era. This is the new Poland, and the new words - of course mostly borrowed from the more wealthy and better West...
I think it's rather for the foreigners. Ambulans is a word which is similar to its equivalents in many languages, including English. And it's shorter than "karetka pogotowia". "Karetka" only sounds very unofficial and it's in fact a diminutive for "kareta" - "carriage" (a type of a horse vehicle). Before the era of cars, ambulances were also horse vehicles, and probably hence the word "karetka" used now for ambulances.
The same is on the firefighters' cars, where it's written STRAŻ only, not STRAŻ POŻARNA, as it's shorter and more visible. Another thing is that they are now dealing with a lot of issues different from fires, but they are still called "straż pożarna", so this is definitely NOT the reason not to put the word "pożarna" on the cars.
I would say that in Polish the word "karetka" or "karetka pogotowia" is used much more frequently than "ambulans".
I presume that other than the benches or chairs of a typcial waiting room there may also be books and periodicals, maybe comptuers and board-games available.
Computers rather not in these times, but a TV could be there.
I think the term "ambulans pocztowy" is rather old-fashioned and no longer used (although maybe the post people still are using it on a daily basis), but... a news from 2002 where it's used:
A more interesting case: panoramio.com/photo/57345101
"Postój dorożek samochodowych" from 1960's. I don't know how to translate the word "dorożka" to English, but it was a horse equivalent of a taxi. So the taxis were initially called "dorożki samochodowe". I think the change had to take place in the 70's.