A long time ago in a galaxy far far away, there was an evil empire that ruled the web with an iron fist. They were accused of being closed and authoritarian. Their latest decrees for the future of the web created rebellion among the web's princelings.
And with that the princelings set up the WhatWG and declared that they were fully open to all and not authoritarian like the evil empire.
And as time went on, the WhatWG set up one and only one author-for-life for their standard. All netizens are equally able to contribute, but some netizens are more equal than others.
And as the web progressed, not because of the work of the WhatWG's standards, but because of a proliferation of new platforms and a bit of still unripe fruit from the evil empire (media queries), there was born Responsive Design. And hacks proliferated on the net among the princelings' fanbois to accommodate the proliferation of new platforms. Among the hacks were ways to handle responsive images: serving different images optimized for the different capabilities of the new platforms. And lo, the princelings' fanbois were all front-end developers, so the hacks were all built around client-side rendering. Did they suggest the clients send new information to the server and let the server-side developers write code to simply handle the issue with simple if-then statements? Of course not.
But wait, the princelings' fanbois were brushed off by the princelings. They were told to go back to the evil empire and act busy. And so they did and the closed evil undemocratic empire allowed the fanbois to establish themselves on their own piece of territory. And the fully-open and egalitarian WhatWG politburo permanent standing committee conferred among themselves and decreed their own Not-Invented-Here solution for responsive images. And there was much wailing and gnashing of teeth among the fanbois, and the PSC's astroturfers went forth and proclaimed in comments across the web, the browser makers have a veto and you should delight in this. The junta has saved you from yourselves and provided an easier solution, so rejoice in your glorious leaders' wisdom and foresight.
Why do juntas do what they do? Because they can. And then they rationalise their actions to themselves and others as being benevolent to the peasants they rule over.