If we only did what we "need" to do, we wouldn't get very far in life! Whilst the bare minimum for university entry is often 5 'good' passes at GCSE inc. English and Maths, this obviously isn't particularly impressive and certainly won't make you stand out. For some courses and certain universities, 5 'good' passes at GCSE are not enough. On top of that, it's not all about university entrance. I feel that I've learned a lot from doing a wide range of GCSEs, even if they're not 100% necessary. Now, that's not to say that school's don't go over the top. At mine, if you take two languages and you're in top set Maths, you'll leave with 12 GCSEs which I think is a little excessive (albeit manageable).
Most schools do only make students take 3 A Levels. You're encouraged/forced to take 4 AS levels for two main reasons. Firstly, it gives you flexibility about which one to drop at A2. If you go into the sixth form with a plan to do just 3 AS levels and continue them all to A2 but you fail one of them or hate one of them, you'll have a problem. On top of that, most students should be able to cope with doing 4 AS Levels if they intend to cope with the workload at university and it shows good time management skills to universities. Finally, there are some universities that require a fourth AS. Again, there are some schools that do for some reason make students take more, but these generally are not the top performing ones. A lot of grammar school sixth forms actively forbid people from taking 5 AS Levels.