Let's face it...it was only VERY recently that ANYTHING heavier than air flew...particularly in the context of "human history". Indeed, there are STILL survivors of the "before heavier than air flight" days. We haven't even "turned over" a whole generation yet. In general, then, EVERYTHING we've ever done to get from A to B (walk, run, ride a bike, drive a car, whatever) has been done, in one way or another, by "pushing off the ground".
thats the reason
people relate a plane to something they know, maybe a car
they know if a car were to be going on a tredmill 20mph and the tredmill was going 20 mph in the other direction that the car wouldnt move
they try to relate this information to planes
daRock.... edited to fix the quote parameters so the quote would show up as a quote (using the quote button above the window works perfectly).