Actually, tbey do change with time. Or at least properly shaped and installed ones do. They respond to gravity over time - not at the same rate or in the same way as the natural ones, but they do respond to gravity - underpinnings you know.
I have saw one woman over the age of 65 with fake breasts, and she looked like a freak her skin had changed over the years as well as gravity and she had 2 bubbles on her aging chest, she looked like a circus animal...