In an ideal situation, yeh, everybody would get the same amount of loan or every person would get assessed individually on need but considering the amount of people who dont end up paying it back this isn't really a viable option due to the huge extra cost this would be. The thing is, for the majority of students whose parents earn more their parents can easily give them the extra money they need for uni, and in most cases a lot more.
Yeh a few people unfortunately are negatively effected by this due to their parents not being willing to give them extra money or not having it avaliable, but this is a small proportion of people. The best solution would be that these people get assessed on a case by case basis, but once again i cant see this being viable option due to the extra time and money that would be needed for this.
Ultimately, very very very rarely is someone not able to go to uni due to their parents income - they still get a small loan and ultimately most parents do end up being able to give them extra money, by like cutting back on things that lower income families already dont do (e.g. Shopping at waitrose or something). Or you can get a job - e.g. I work all summer full time and part time at uni to make up the extra money i need, i dont get any help from my parents.