Why is my hamster CONSTANTLY trying to escape from her cage?

My hamster, Daisy, is spoiled. She has a big 20 gallon tank as her cage, with plenty of food and water and treats. She has a lot of toys to chew on and paper rolls to climb through, and she has her own wheel. But what puzzles me, is that she is constantly trying to escape! She literally climbs to the top of her wheel and tries to chew through the mesh. And she's actually chewed the black rimming on the cage! But whenever I hold her, she's happy and content. She snuggles into my hand and falls asleep, she's never tried to run away. But I just don't get why she tries to escape. Is there anyway to stop this behavior?