Not all characters need
a backstory, it depends on the character in question. Now, think about Cozy Glow for a second and question yourself if she actually has
to have a backstory or reason for villainy.
In almost all of her appearances, she manipulates/hurts others to get what she wants, and sometimes for no reason at all, and in the of this last one, she already distance herself for other reformed characters like Starlight and Discord, as these two, even if they didn't have backstories, still had a "reason" to do things, some more "reasonable" and "logical" than others ( a misguided attempt at creating an equal world
; Chaos for fun; etc.).
However, that's not necessarily because of a failure in writing, it is — and this is my view on the matter — because Cozy plays a different type of roles than other characters. Basically, she's a psychopath, and even if she doesn't always match the medical definition to a T, her actions and behavior just scream "Psychopath" and everything involving her in the show almost always point in this direction, for lack of care in others that don't interest her, to her manipulation, etc.
Cozy is a psychopathic, megalomaniac filly who hides her true self in a cute facade, manipulating other in clever ways for whatever reason until she no longer needs them. That's the point of her character
. Giving her a backstory just for the sake of it or because other antagonists had them would be redundant as it would only reaffirm trait we already now she has without putting them in a new or interesting light, and since she does what she does because she is a psychopath, she doesn't have a "motive" (other than a lust for power), so a backstory exploring that would be a waste of time unless the writers had not decided that one of her primary traits was Psychopathic tendencies, which nullifies ( or at least, it should have
, but she's a cute filly, so…) any degree of sympathy the audience could have for her.
She was given chances to stop and to explain herself, but didn't took or desired them. She did cruel things, sometimes for no reason. Why does she need
a personal and detailed backstory to build on that, if she herself already does that?
That's not to say that giving her a backstory would not
add something of value to the her character, the worldbuilding, or whatever else, but if it is not needed due to the role that she plays, then calling it a "lack of effort" would be missing the point.
I'm also not saying that I or anyone else doesn't want
or wouldn't appreciate
a backstory for Cozy or that they would be in the wrong in wanting it. Hey, more pony content is
never bad, and a lot of potential for interesting scenarios could be hidden in there. Heck, I
wouldn't complain about the inclusion of a backstory for her. But, as controversial as things are, it's fine (at least relatively) the way it is. Better that other ways things could have gone, that's for sure.