Just a guess, but I'd think the most common cause of broken links is the original administrator retiring that whole branch of the site (or the site itself). Often the domain still works but that old stuff has been thrown away.
Often it's clear that what's there now is a lot better or more professional, so you can see why the person didn't feel like messing up the site with the old stuff - but that old stuff is still gone.
Other times the domain is gone, since the person or people have moved on to doing a lot better stuff and stopped maintaining that old site - "why bother." People change - a web site isn't something you publish once, it's something you publish every time your server answers an http request. Would you keep publishing everything you wrote 10 years ago?
Often it's clear that what's there now is a lot better or more professional, so you can see why the person didn't feel like messing up the site with the old stuff - but that old stuff is still gone.
Other times the domain is gone, since the person or people have moved on to doing a lot better stuff and stopped maintaining that old site - "why bother." People change - a web site isn't something you publish once, it's something you publish every time your server answers an http request. Would you keep publishing everything you wrote 10 years ago?