Vapor from gas/diesel/jetA is heavier than air and therefore flows to lower elevations, which is why inboard power boats have bilge ventilators.
An elevated antenna is likely much less exposed to fuel vapors than a mobile at ground level and likely the engineers have worked out a formula for a safe antenna elevation.
Propane and methane fumes of course go up; so please don't mount your antenna on top of your propane tank unless you want to send it in orbit
