Why does anyone require a solo film? With a good enough writer none of them do. Superman? Doesn't need his own solo origin film. He's not a particularly complicated character, nor is his backstory- a good writer could flesh him out in an ensemble film quite well with just a few scenes focusing on his backstory and pathos.
The only reason any comic book character gets his own solo film is because they're famous enough that the producers think a solo film will sell. The only reason Warner Brothers isn't giving Diana a solo film is because they're too dumb to understand that a comic book movie with a female protag would sell so long as it was actually good, and they're tripping over themselves trying to catch up to the MCU.
Basically saying "I don't care if one of the most iconic characters in the entire DC universe is fleshed out" is fair, but then you should say the same about any other character then. If Wonder ****ing Woman can't have ten minutes explaining her origin than none of them should. Batman? "I'm a rich guy who fights crime." Flash? "I'm really fast." Cyborg? "I'm a minority lol." J'on? "I'm an alien." Green Arrow? "I'm Hawkeye."
I don't mind that line of thinking, so long as it's applied across the whole cast. I mean, the movie's probably gonna be ass anyway, so why not have a film centered around "characters" where none of the characters are 3-dimensional.