Because online school helps you like you can learn from comfort of your home. They can also have teacher teach students by computer more than going to a school to teach. Online schools can also give less more homework than traditional schools. Online schools can help students learn from compuers.
In traditional school, students deal with real live people, and interact with other human beings in real time, as opposed to over the internet. Through this a child learns his/her subjects better, as they have the stimulus of a teacher's voice and all the non verbal communication that they'd be missing out on if they learned online, and as they have (healthy) competition with other students in the class. In addition, they also learn how to pay attention, how to read another person's body language and how to get along with someone of a different background and upbringing - basically they learn how to interact with other people, something they wouldn't learn from working at a computer.
Getting less homework is not a valid argument - the amount of homework a student gets would naturally depend on several factors - who his/her teacher is, what course she/he is following, what age group and the country he/she is in. These factors would be equally applicable to online or offline education.
Additionally, school is not just about studies, it's also about all the things that come along with the academics - the societies, sports, leadership opportunities and the friendships made. These are also an integral part of a child's education. Therefore, traditional education is more effective than online education.