Why do satellites burn up when it hits the atmosphere?

Because before they hit the atmosphere they are travelling at extremely high speeds without any resistance (from wind). Therefore when they enter the atmosphere the wind resistance builds up and creates friction which also brings lots of heat, causing them to burn up before they hit the ground. This doesn't just happen to satellites; most if not all objects travelling from space into Earth encounter this. The only exception would be space shuttles because they have been specially designed with heat resistant panels etc.