Bezos and Amazon are among the four super-powers of the internet for good reason. I imagine they've run cost/benefit analysis and found the ROI of investing and focusing on any their new frontiers (AWS, Kindle, etc...) to outweigh the ROI of a redesign at this point.

But funny all the same :)

* They had good UI designers to start with, so no major changes were needed.

* By now the site appearance is part of their brand, so they don't want to change it.

* Amazon may now be such a complex site that a change would be expensive.

* It's not broken, so there's no need to fix it.