I have passed over downtown just one time that I can think of, on a flight from Seoul to LAX. I've seen lots of flights into LAX coming over various parts of L.A. I had the impression that flights out of LAX generally took off over the ocean and then made a wide arc if going east, making a pass over d.t. less likely, but your observations seem to counter that impression, at least in some cases.

If you're flying into or out of BUR from/to the north or east I think you'd be unlikely to pass over downtown. Ditto for ONT. Although exact flight paths do vary according to each day's conditions, I can't think of any flights into or out of the latter two airports that would pass over downtown.