Yes, preparation makes a big difference in math olympiad but most of those competing at a high level are already very talented to begin with. The problem is when a parent expects their kid to achieve similar performance when they don't have the raw skills. By attributing no role to inherent ability, blame is always put solely on the child for not trying hard enough. Even within a given family, you'll notice that some kids are better much more talented than their brothers or sisters at a given task, and the difference is not really preparation (especially since these differences can often be seen at a very young age).