As someone pointed out, the engineering required to harden those (against thieves, vandals, weather) is impressive. Thus, the amount of money your cities, universities, airports, etc. pay up front for paystations and then annually in mandatory maintenance contracts (that's where the real money is made) ... ugh.
I'm not saying our UI & UX over the years are without flaws (IVR, WML, SMS, & finally apps) but at least you're not standing in the weather or leaving your appointments to feed the beast.
† Added: Actually, the best payment interaction might be no interaction but the whole always-connected-and-location-aware-car wasn't on our roadmap in 2000. The scene is ripe for change again IMO.