To get a well rounded education.
why do we need to study different subjects?
To give you a broad spectrum to find out what you naturally gravitate to for a specialized field of learning.
This ensures that you enjoy what you do for a living as well as excel at it.
Because the world around is made of many those different subjects.
Because the different subjects will provide you with different kind of knowledge which is required to undersatnd the world
Because being a well-rounded person with a variety of knowledge makes someone more interesting and more open minded.
To find your strengths and work towards them. A lot of the subject matter you will forget when you get older but you will hold on to the capacity to learn forever.
Einstein - "Education is not the learning of facts, but the training of the mind to think."