Because- for instance- in the book I've just been reading- The Stain on the Snow- he really does try to get under the skin of his 18 year old street thug (how he's comfortable in some places, uncomfortable in others- afraid of some people- not necessarily the obvious ones- how he calculates his bravado, how he's not as completely in control of himself as he thinks he is). Maybe the exercise doesn't quite come off and the psychology is a little too pat (thank you for those insights, Dr Freud) but at least here's a writer who's working his imagination hard.
With Leonard what you're getting- for all the accuracy of the surface detail- is romance. He's into the whole Chandler thing where the street-soiled gumshoe is really Sir Galahad and the crim is a laid-back mastermind and the gal is one hell of a classy broad. It's like he's wetting himself that these people carry guns and it's all so cool.
Simenon wants to give you some truth and Leonard doesn't. Simple as that, really.