What's the point of maintaining a CV after tenure? Once you have tenure, presumably you're never going to have to search for a job again. However almost all professors have a CV on their website, and fairly recent ones. Why? They already made it.
Upon joining the club of those who've "made it," you find that there are further gradations.  A tenured professor can safely breathe a sigh of relief for at least a year or two, but eventually she will notice (i) that her salary is lower than most of her tenured colleagues and (ii) that she is still being asked to document and defend her productivity across several axes: research, teaching and service.  (Also, in many cases, (iii) She is increasingly pressured to apply for and get major grants.  In many cases, her future salary and promotions depend on this to a large degree.)

The main tool to exhibit your recent accomplishments in research, teaching and service is...your CV, usually.  In fact, "whether they want a good raise or not," most faculty are required to submit annual "activity reports" in which a CV is often a required component.  (When it isn't, it usually means that we manually copy information from our CV into some form or type it into some internal webpage.  So having an updated CV is more or less necessary for 
many faculty members.)  Certainly having a carefully groomed CV is necessary when you go up for "promotion to full", apply for an endowed chair or distinguished research position, and so forth.

The bottom line: academia has bureaucratic, competitive and hierarchical aspects all the way through.  Most people who have made it tenure in recent years have learned how to keep reasonably up-to-date CVs without too much trouble or heroic effort -- the CVs do not need to be rewritten from scratch to showcase their skills for some new job, usually, so it is mostly a matter of regular updating -- so honestly, this is not the worst part of the bureaucratic minutiae.