So far as I can see, there only a few simulators that allow users to create their own environments or "players" (e.g. trains, ships, aircraft, warriors etc). The vast majority are not user programmable.
In Ship Simulator, users can and do create missions (which are incredibly variable in quality), radio sounds, horn sounds, vessel skins and ships lighting.
I'm very familiar with environment creation in simulations and the vast majority of people will probably never be able to, or indeed want to, do this. Even fewer will be able to, or have time to, make a really good environment.
There is of course a design group section in this Forum, so it isn't blocked to "outsiders", just well-controlled.