Some possibilities: * 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

Some possibilities: * 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.