"Why is this a thing" is a stock question used to express confusion and/or exasperation about a recent cultural artifact. As far as the actual query expressed it might be better phrased as "why does this exist" or "what chain of events could possibly have brought this about" but the feeling of bewilderment and disapproval is better expressed by a pseudo-tautology. This is a blatant internet slang mostly aimed at bizarre trends promoted into existence by the mysterious algorithms that control the world. A few examples:
Minecraft parkour footage with humorous dialogue dubbed over it.
Broken NPC dialogue spouted by streamers.
Everything to do with the Undertale Alternate Universe fandoms.
Flat Earthers.
The live action Dora the Explorer movie.
Buns coming in packs of eight while hot dogs come in packs of six.
While it's easy to apply this to anything weird (and the slang police will not bust your door down for doing so) it's mostly reserved for multi-participant oddities. Lone oddballs creating and disseminating their brand of strangeness over the internet was a driving force in it's early adoption and has remained a persistent element into the present. One persons quest to document and comment on anime bathing or attempt at an elaborate fantasy life through their own Wikipedia page is just normal levels of peculiarity in the broad scope of five billion internet users. "Why is that a thing" is about inexplicable moments when everybody starts talking about eating Tide Pods. A lot of trends are conceivable in origin and appeal even when they aren't your cup of tea. Others not so much. In our bafflement we cry out to the digital void for context. Sometimes it answers.
Google trends indicates that this phrase has been growing in popularity for over a decade but with most of it's growth in the last few years. One might go meta with it and ask: why is this a thing? Is the world getting weirder? Does it just sound right? Is English devolving? Perhaps all three.
IRON NODER XVII: ALL'S FERROUS IN LOVE AND NODING