Way back in season 1, they were. LOL. But, they have to keep upping the ante as time goes by, or the show would get stale for the regular viewers.
I watch an embarrassing amount of true crime shows (so much so, that I can recognise the cases when two different shows do it at different times), and from what I can tell, there are some really devious, sophisticated killers out there who have almost gotten away with it. But, yeah, the norm is simple killers, with simple motives who are not half as clever as they think they are.