Nobody expects the physics in the game universe to be different from that of ours, but we can reasonably expect ideas which are sustainable in that universe because of the existence of other supplementary concepts. I imagine that people who needed to design and build a mobile city were able to do so because they had the right idea and the right stuff at hand.
Essentially, saying "if it does not exist right now, then it can't exist ever" is just incorrect; you can't say that something is impossible when no one has tried to do it. Do you think if people were tasked with designing a mobile city, that they would come up empty? Most importantly, you can't apply the restrictions of one scenario to all imaginable scenarios, that's also wrong.
Actually darlin, we CAN expect physics to work the same, and in fact will be quite dissapointed if they don't. The beauty of Cyberpunk 2020 is that the game designers payed really close attention to how things worked. They made a few leaps, but nothing beyond the realms of plausibility, in fact nothing that really even stretched plausibility all that much.
Even if hypothetically, you could uild your massive mobile city thing, where would it go? Bridges and overpasses on highways would prevent it from traveling on roads, uneven terrain would prevent it from moving off-road. The only place it could really function in one the great salt flats of Utah, a place where there is no food or water to be had.
It would also, by necessity, be extremely slow.
Take for example the Crawler, used to transport the space shuttle to the launch gantry.
Close to 3000 tonnes,
each crawler had 16 traction motors, powered by four 1,341 horsepower (1,000 kW) generators, in turn driven by two 2,750 horsepower (2,050 kW) V16 Alco diesel engines. Two 1,006 horsepower (750 kW) generators, driven by two 1,065 horsepower (794 kW) engines, were used for jacking, steering, lighting, and ventilating. Two 201 horsepower (150 kW) generators were also available to power the Mobile Launcher Platform. The crawler's tanks held 5,000 US gallons (19,000 l; 4,200 imp gal) of diesel fuel, and it burned 125.7 US gal/mi (296 l/km; 104.7 imp gal/mi).
It travels at 1 mile per hour. And it's entire life is spend driving back and forth on the same 3.5 mile stretch of reinforced road, which it takes an average of 5 hours to do one way.
Or take the equally impressive 360 ton Caterpillar 797. Yeah you could fit a small village on it, and it's even off-road capable... kind of, they can even make it up to 40mph. But even they are pretty limited in where they can go. Nomads do use these as mobile bases, but this is about as close as you are going to get, and these things are pretty limited in where they can go. And if they get anywhere near a city, then air strikes are called in and the whole mess goes up in flames. I wouldn't mind seeing something like this out in the nomad areas, but understand, it can't go out on the roads with out destroying them anytime it comes near a bridge or overpass, which would get the corporations and Government pissed enough to destroy it utterly. It can't cross bridges, it can't cross mountains, and it can't navigate on wooeded backroads or though narrow passages. It can't go on soft or wet ground, because it would sink. And the maintenance require to keep it running would be nothing short of monumental. Now it's pretty solid on armor, but the wheels by necessity are going to be vulnerable... and you don't want to imagine the logistics of changing a flat on one of these... it requires 6 of them, and each one costs 45 grand.
There is simply only so big something can get before it is no longer viable to travel over land effectively.