A lot of it’s structural. Most (not all) of life is episodic, starts-and-stops, with comparatively few scenarios in which a dramatic situation or crisis presents itself and works steadily to a crescendo in which most or all of the loose ends are involved.
Not only does that make it challenging to build a believable plot and nuanced characters – it also makes it almost inevitable that by the time you’re most of the way through, you’re going to have a decent idea how it will play out, unless the author resorts to really cheap or unbelievable tricks or unfair tactics (I’ve always thought the best thing to do in a murder mystery would be to have the final line be “. . . and then it turned out the killer was an unknown hobo who had just been passing through town.” Except not.).
There are also only a finite number of plot twists, motives, and outcomes to combine.
I kind of liked Wambaugh at first because many of his novels just ramble loosely between episodes involving an ensemble of characters going through their days, sometimes not really even reaching anything you’d call a single denouement. But his books do get very samey, and it’s unfortunate he was writing in the late '60s/early ‘70s. There’s a bit too much of a mod squad/disco vibe to a lot of the themes and characters, though the noir description of the cops’ attitudes is pretty spot on (at least in terms of what the genre portrays; Connelly certainly owes a lot to Wambaugh).