Really good question!
I live under a major flight path inbound to detroit. some days I see many contrails, other days I see a couple or even none at all. Most of the time I see several jets but not all are leaving contrails. most likely due to the fact that they’re flying at different altitudes.
Its also important to note that the ammount of sky we can see is enormous. If we see two planes at opposite horizons, they are a long way apart