Flash happened to be born and develop its feature set in a rather dark time for security, the mid-90s. In this time period, it was considered a good idea by marketing to include a fully-featured general application environment in everything - web browsers, video player formats (Quicktime, WMF), plugins (Flash, Java applets), documents (VB for Applications), etc. For the most part, a large number of these have reconsidered those ill-considered plans (browsers, Quicktime and WMF) either by neutering the languages they had embedded or properly instituting a solid sandbox around them (Javascript is the poster child of successful sandboxing); others have faded away to disuse (Java applets are still supported but rarely ever used, even though Java the language is still going quite strong outside the consumer/browser space).
That leaves the bete noires of the security world, the deemed-critical-by-some technologies from that unfortunate era which just refuse to die. Flash has a sandbox of sorts, just not a very good one. It has bugs, but the remarkable thing about it is that those bugs tend to not just cause the app to crash or do something unexpected, but to open up the entire machine for remote takeover without any user interaction whatsoever. VBA is in worse shape - Microsoft hasn't even ever thought about instituting any kind of a sandbox there or putting the more dangerous features behind an optional install, etc, and so we see MS "word macro" viruses still popping up twenty years later.
But back to Flash, how is this different from glibc etc? glibc bugs are scary, but they are tempered by the fact that there is no other way to do what glibc does, while there are healthy alternatives to almost every single use of Flash and VBA which do not require a full-featured general-use tool with full system-level access. Yeah it sucks having to rebuild and reinstall apps because a common library was found to have a vulnerability. But at least you have specifically chosen those apps for the thing that they do (and, to be clear, the only apps you really needed to recompile with the glibc bug were those which resolved internet addresses using domain name lookups; other apps were updated with the latest glibc but just to keep current not as any kind of an emergency update job). Flash might be chosen by users - and almost always remains on systems simply because of this - to play video, but along with that Flash includes massive amounts of features which allow a bug in any of that code to start a system takeover.
Why the hate for Flash? Because if someone has been bit by a security vulnerability, chances are it came from Flash. Because that is the one piece of software that we keep around for one or two web sites which haven't stepped into the 2010's yet and moved to single-purpose safer alternatives, which is constantly needing emergency updates.