A couple already do have pseudo-simulators on them; Project Wingman, for example, runs on Unreal (ignoring that it’s more of an arcade game, but a lot of the same principles apply).
In the past, the usual limitations were typically the physics engines and world space. Some engines also had difficulty with joystick support (I think it was Unity). When you put them all together, and coupled with the fact that sims are niche and expensive, you end up with folks like ED taking their core engines and just transitioning chunks and pieces over time. If the physics core works well enough, there’s not much reason to ditch it all just because the graphics engine is 5 years out of date.
Another thing that is seldom considered is the passage of time and how it affects lighting. A lot of the above engines are designed for one time of day at loading of the map and can’t be changed dynamically (though there are workarounds, they are not incorporated by default). I’m not sure if the latest Unreal engines work around this yet, but a couple years back there were several hacks to get time of day and the resulting lighting and shading changes into the engine that were imperfect.
Lastly, physics are often the real troublesome bit as most of the above engines use simplified physics for a lot of things (which makes sense). When you try to incorporate a flight engine – which is pretty demanding – it has a tendency to bog the engine down. I recall reading a couple threads about that on Unreal development where they ran into some problems of getting the flight engine updating as quickly as it needed to be.
Being fair, a lot of these issues are typical of most game development, but they take time and money to solve, and games aren’t real cheap to develop to begin with. Flight sims are already a very niche market and if a new developer is trying to compete with ED, 777, etc. they’re not liable to get dollars if the rivet counters aren’t made absolutely happy. Likewise, if you can’t capture the average casual gamer, you’re not going to do to well there either. It’s a very rough tightrope to walk and that’s why you get things like ED pushing the bug module – a well-known and popular aircraft is a great way to get some sales numbers up. Flip side of that is you have to get a team of lawyers in place to make sure your license is good to go.
At the end of the day, it’s not impossible (I still keep an eye on Unreal every now and again for a potential Longbow 3 project), but it’s not an easy undertaking either. Hence, most folks with the skills and ability to make games will do simple stuff to pay the bills.
P.S. Check out Empyrion or No Man’s Sky if you’re looking for the all encompassing simulator.