Im a F/A for UA and I understand exactly of what the OP is stating. I tend to notice it especially strong on the 757, but no aircraft is immune.

Im not sure why other posters have implicated other aircraft, as the engine exhaust closest to the OP would be their own aircraft!

When on taxi before takeoff, the cabin is not in a fully compressed state so air travels freely between inside/outside through the door seals. This is how pilots have explained it to me in the past.

I notice the smell is especially bad on first start-up after the aircraft has done an overnight.