Intellect grows over time. A four year-old, for example, though more intelligent than a baby, is still very stupid. A nine year-old is very smart (at least compared to apes whose intelligence capacity peaks much earlier), but nowhere near as intelligent as he will be as a 14 year-old, and there is another huge jump between 14 and 18. The structure of the curriculum in different phases tries to reflect this. We generally teach simple things that to a large extent can be rote-learned in primary school, then introduce some logical thinking in secondary school and only expect real analytical and critical thinking in 6th form.
You are probably quite smart and so ahead of the curve, but the national curriculum is pitched at whole year groups and the vast majority of your peers would not be able to keep up with you. That said, I do think the content of the curriculum at KS3 could be improved, but the level of difficulty is probably about right. I think the problem that you experienced is probably more due to the fact that "teaching" methods have shifted towards "facilitation", where children are supposed to work things out for themselves. I think that could generate a feeling of "I didn't learn anything" because as an approach it is more skills- than content- or structure-oriented. (And naturally children like to do things they already can do, and (most of the time) not things they can't do.) I think a "marginal gains" approach would probably be more efficient, and you would feel more like you are actually learning something / making progress.