Just to be clear! Are those you mentioned not rather fighter-flying-games or flying-action games, than they are simulators? I mean.... Simulators simulate something in the real world, and the goal is to reproduce some kind of higher level of AI compared to those you have listed? Like, they are somewhat less of of simulators, compared to a real simulator that are kind of leagues above those you mentioned. Like "Flight Unlimited", "MS Flightsimulator" or "X-Plane". Those are proper simulators, and not mession based semi-simulators. Not that they are bad games. They are freaking awesomme. Yet they are more real game's than they are real fullblown simulator's. Are they not?
Just to be clear, these are old games, at the time they were considered flight sims. Even the ones you brought up might not be flight simulators by your standards. Flight Unlimited only modelled very good flight model (again at the time), even then there was nothing more to do there but fly aerobatics. The terrain was very limited - only several km^2 areas around single strip. No navigation, engine management (separate throttle, mixture, prop pitch controls), even no fuel flow! With Flight Unlimited 2 thay came closer to Flight Simulator, AI traffic, ATC, radio-navigation, ILS approaches etc.
When you look at the "X-Plane" (which cost several hundreds $ back then) and Flight Simulator from mid 90s it wasn't stellar either. Besides those I showcased are military flight sims prduced for consumer market. Although few devs after doing their sims did acquired military contracts for doing the REAL DEAL flight sims, like Digital Image Design afer their Eurofighter 2000 released in 1995.
It was just limitation of the PCs at the time. The flight sims got more and more complex as the time passed - look at Jane's F-15, Falcon 4.0 and so on. Even though they were considered complex already if you compare theam to something like Falcon BMS 4.34 or fully fledged DCS module it would look like child's toy.