Imagine this: you are Google. Raking in billions. PhD level scientists are hired to mop floors, and serve coffee to everyone. You hire top level designers, pay them $250k/year and tell them to improve Gmail.
And they do...Gmail gets better. You make more billions, hire more designers, pay them more and ... tell them to work on Gmail.
You now have 10 designers, "designing" GMail. Come end of year performance review all those highly paid, world class professionals will want to say "I did <x>, <y>, <z> and I deserve a raise". Nobody will want to say "Yeah, GMail is actually great so I left it alone and didin't muck with it, can I have my raise?". So, they go to town and re-design it every 6 months. Review comes and well they all can claim they have successfully re-designed, flattened, styles etc, added custom iframe, windows, etc.
This happens with software teams, that is why so much software is re-written, people don't get rewarded for maintaining as much as for creating. So they have to create, and when things get crowded, that means tearing down and then re-create.