A small percentage of kids are probably hard to deal with when it comes to sleep no matter what you do, but most of the parents I've seen with kids who still often wake them up past the 4-6mo range are training their kids to do it. They can't possibly realize they are or they'd surely stop, but they are. This is usually combined with waiting way too late to start trying to sleep-train.
I hate being that guy who asks where you infer it's a "small percentage", but my anecdata are that disrupted sleep is the norm. Yes, it got better after the first 3 months. Then there were multiple - very natural, I hear - sleep regressions: expansive brain growth which led to night talking/nightmares, teething, illnesses, fluctuations in natural body temperature regulation... kids are rapidly evolving creatures, and habits/pattern/training are great to instill, but there are a lot disruptors both natural and environmental.
