Zawinski's Law

Every program attempts to expand until it can read mail. Those programs which cannot so expand are replaced by ones which can. -- James Zawinski, 1995

While feature creep certainly happens in non-commercial software, I have observed that this effect is more powerful in commercial software. My theory is that when your salary is justified by writing new code every day, you have a powerful motivation to find new code to write and new features to integrate. There's no mechanism in our society to say "this is enough software for now" while still paying the team.