I think it's not so much "don't want to" as "don't bother to".
And some of my colleagues "of a certain age" are still disdainful about the internet, and disdainful about learning how to write basic HTML and put documents on-line. Some of this may be a hold-over from a time in which computers were thought of exclusively as devices to do computations, rather than to communicate. Further, some mathematicians were/are disdainful of computational mathematics... And I've had people (now mostly retired) very directly say to me that "maintaining web pages is not part of the job of a mathematician". (I'm pretty sure that these people had ulterior motives for claiming this, but, still, they were willing to say it!)
And it does still appear to be the case that no amount of on-line stuff can substitute (in mathematics, in the U.S.) for traditional journal papers (even if they are hidden behind pay-walls), so, again, some people simply don't bother. Even listing things might be construed as pointless, if all that matters for some purposes is impressing one's department head, dean, and funding agencies.
It is also true that (perhaps motivated by security or economy concerns) many universities are changing to a "cloud-based" web-page model, which makes access much more complicated (from my viewpoint) as opposed to the straightforward (from my viewpoint) access via a unix/linux-style file-system allowing simple movement of files from one's home directory, for example, or via "scp", and remote access via "ssh". Indeed, it has taken significant effort to at least temporarily prevent this kind of change in my department, for faculty web-pages, and it was irresistible for departmental and grad-student pages. I'd wager that before I retire, it will change for the worse... Then it would become much less simple to update and change things.