If they don't simulate flying a spacecraft, they aren't space sims, but simple shooters. Not sure if space because space in them certainly doesn't behave like space.
You can't simulate "spacecraft" because "spacecrafts" like in your generic space sim don't fucking exist. Space navigation as we know it today is very limited and very dangerous and expensive. You can't "simulate" that which doesn't exist and will probably not exist either.
What we consider a spacecraft today is the kind of vehicle that can break apart if a piece of foam hits one of the wings. Space travel and space stations as we know it means going into low earth orbit , only a dozen people went further than that and actually visited the moon. Everything else was unmanned and even those haven't completely left the sun's orbit, in a travel that took them decades.
Here are a few images to help you understand the scale:
The scale of low earth orbit space (the place where manned space travel takes place) compared to the moon.
How giant is the distance from Earth to Mars compared to the distance from Earth to the Moon.
How far did a man-made object managed to travel so far. The oort cloud is technically the "edge" of the solar system. Voyager 1 will shut all of its systems before reaching that.
And that's the basic part. You better hope that once we reach Mars in a couple of decades, there's some magical "quantum physics hyperdrive" from ancient magical alien race that allows us to move at FTL speeds (which is mostly physically impossible), ignore all the debris and radiation, ignore time dilation and plenty of other things that make space simulators as we know it simply impossible beyond any measure of doubt.
And that's the logistics on reaching other stars alone. Then you have the whole idea about colonizing and terraforming other places, living in space which is hard enough for psychologically prepared astronauts let alone normal guys, and all that shit. Making space traveling such a commodity that one guy can just go whatever the fuck he wants in his one-manned ship, like it's some kind of a car. Allowing individual people to just "mine" and "trade" in space just like that. Thinking that piracy in space would be really possible.
There are so many things that are not realistic in "space sims" it's not even funny. Here's how a true space sim would actually work:
- convince lobbyists and the government to fund a new set of manned missions to the moon.
- defend yourself against a butthurt Buzz Aldrin who will argue that you will ruin their legacy by going to the moon again
- try not to cry when the government only gives you the middle finger.
- debunk conspiracy theories about the moon landings being a scam on your website, using an MS comics font
- try to convince an old fart to spend a few tens of millions of dollars so he can go to space on some derelict russian space station from the soviet era
- be ass-devastated that you're not a little snowflake anymore now that china, europe, japan and india are in the space race as well
- try to contain the need to shit yourself the moment a couple of astronauts are launched in space from the fear that the smallest of things could have went wrong
- quote Hawking, Sagan or Neil deGrasse Tyson on reddit and manage the "SPACE IS AWESOME FUCK YEAH!!!11" facebook page where people post space-related memes
- send another little robot on Mars
There you go, enjoy your space sim! Now fuck off and let the people who enjoy fantasy adventures in space have their fun.