Let's take some examples. Some parents invest hugely in, say, music lessons, riding lessons, tennis lessons, foreign language lessons, ballet lessons. I know adults who as children spent hours, week in, week out, practising something, for years on end, and achieved nothing at the end of it (except hatred and regret for the wasted opportunity, when they could have been doing something they enjoyed instead). Not all children can achieve anything by any stretch.
First off I believe all children can achieve.
Secondly, did the children want to do these activities? Did the parents find out what it was that drove their children, what was their passion? Were the parents trying to fulfill their own aspirations through the children?
Did they sit down with their children, even when they were very young, and explain how learning was fun, learning was a journey, sometimes tough, sometimes they'd have to graft. Did they explain, in a friendly non-threatening way, what the purpose of it all was?
Did they support the children in the 'right' way? All children can achieve, how can you say otherwise?