Why Can't People Figure out that David is the Hulk?

It is really annoying to me the way that on this show, no one seems to understand that David is the Hulk, even when he transforms right in front of them, for example in 747 when he tells the others to leave the cockpit when he knew he was going to transform, then the kid comes back a few minutes later and asks 'where is David? How are you?' Come on, how hard is it to figure out that he IS David? This happens a lot in the series, where David is being attacked by thugs, and transforms during the fight and the bad guys so often respond 'where did this monster come from, where the guy we were just beating up go to?' Come on, is it that hard to figure out?