I think it is because not enough is expected of pupils. People are put in comprehensive schools and therefore are not required to perform any better than, say, the second-from-last quartile. It is the lower and upper quartiles that suffer, one because they're not intelligent enough to learn even basic arithmetic (although this need not apply to a huge number of people), and the other because they are of course held back by the modal average. What makes things worse is that ability is obviously positively skewed, which is to say that the mean ability is stretched way above the mode, since some small number of people are brilliant, far better than average. What this means is that creativity and intelligence from the brightest students is actually stifled, which is terrible and creates a kind of aristocratic social stratification where only wealthy private school pupils are allowed to fulfil their potential. This is an unfree system.
The answer is to place people in different schools depending on ability. It is more humane on everyone.