Because people need to be educated. You want to grow up ignorant and stupid? Where else are you going to learn the things you need to know to interact intelligently in our high tech society and imoress an employer enough to get a good paying job? Reading and writing and speaking intelligently dont come automatically. They have to be learned! Unless you WANT to be ignorant and look forward to working at McDonalds or Walmart the rest of your life then a good education is very important and school is necessary to do that.
There is school because it is in society and the governments interest to have a literate and educated population. The more knowledge you have the less likely it will be you will be a drain on society either criminally or financially. The more educated you are the more likely you will have a higher paying job and contribute more money to society in the form of taxes and creating jobs, etc.