While that's true, the reality is they could make an excellent guess if they wanted to. Make an F-15C sim, change the FM to be as good as we know the F-22A to be (its real performance is classified, but just plug in the abilities that we KNOW and then guesstimate the rest is improved by say 20%), put in "god mode" radar/sensors and you're done!
Is that "realistic", no, but for all intents and purposes it's as close as you can get in the unclassified world. To just NOT make one with the excuse "we don't have accurate info" is crap.
BTW--I will point out we've had sims (albeit not hardcore) with the Su-33, F-22 and F-35 in it in the 90s but they've not bothered to make one in the 00s other than LOMAC which just pretty much carried over the 33 from F2. We've not had ANY sims with them, hardcore or otherwise, in the 00s. The best has been console/arcade stuff like Ace Combat or Over G Fighters which aren't sims. At this point I'd take a lite sim like TW's stuff in a heartbeat.