That's part of it. Another cause is that companies grow and get more employees and those employees need to justify their existence by changing things up. You don't get promoted for not fixing what aint broken.
It works the same way it did in 1999, so my grandma can still sell stuff on it. That's it. Done. It's probably made way more money by not changing anything than by increasing CPMs 5% by playing a fullscreen ad every 6 seconds.