There's 1 company theme but many different web applications with different theming approaches, for e.g. mailing list software, issue tracking software, CMS, blogging s/w, etc. The company may wish to use for e.g. WordPress to manage their blog rather than whatever comes native with their CMS. Management may decide for e.g. to move to a new CMS platform. Upgrades do tend to also mean rewriting applications to work on the new version and this can be a long drawn out process. And usually by the time you've completed it the new version of the software has been released.
Uses standard HTML and CSS. No need to integrate the theme into the web application technology. Can use it as is. - Can either be run as a separate proxy server, as WSGI middleware or using an XSLT inline transformation hence allowing for better performance and flexibility.