The reason parents know everything because you are their child and the know everything about u. Your parents will know because you came from them ad they know the same things they did when they were your age. If parents don't know how you act daily they could never know how you will be.
Although they keep saying like "I've been a teen before" or "I understand", sometimes they just don't! Yes, they've "been there", but how long has it REALLY been since they've been teens? I remember them saying that people were pretty nice back then; to have a friend, all you needed was good grades and be a good friend. Now in the 21st century, you have to be popular and have money and the latest phone to really have a friend that sticks with you, and if you had good grades, you either got called a nerd, or people befriend you to cheat off of you! Plus, not only that, even if parents ARE right, they don't listen to their own advice! "Clean up!" they say, and you see their stuff all over the floor. When you ask why, they said they "were busy". Hello! We teens have a ton of homework to do, all due by a certain date, and you parents can adjust your schedule anyhow to clean up a simple mess! I'm just saying this because this is the way my parents are; I am aware that some parents can be fun and TOTALLY understand the ways to the teens somehow.... Not spoiling their kids, but at the same time, sympathize with our situation....
Nobody knows everything, let alone parents. Parents are there to support you and have give you advice based on experience that they have had when they were in your age or shoes. Parents may know a lot more than you, but definitely don't know everything about everything. Some parents may think they know everything, but they themselves need to read this.
Being a parent myself, I may want to think that I know everything, but it the end, no one really knows everything. Sometimes to a child it may seem like parents know everything about everything as well as see everything, but this is not true. The mere fact that a parent was once their child's age gives them insights on the outcome of many childhood ventures, but it is far from knowing everything. Regardless of not knowing everything, the wise parent will know where to find the answer to everything!