They are actually a function of temperature rather than just altitude. So, do you want an entire weather aspect to the programming? Because contrails will form at different alts (temperature will vary at a given altitude from day to day for example) depending on the temperature at various altitudes on that day.
This is obviously an unfair statement, though.
Like, landings, for example. Landings and take-offs are the most complicated and dangerous aspects of actual flight. But does AH2 recreate every second of necessary detail in depicting such complicated actions? If a game isn't willing to model headwinds and bearings and stuff, should it not be able to model "landings" at all?
Ofcourse not. Simplified and easy as it is, landings are in the game.
What does it really add to the game? IMO, not much. It'd be a waste of bandwidth IMO, especially considering we mostly have radar.
Visual realism. "God's in the details".
It's a very, very powerful and compelling tool.