IMO, if they went that route, it would've been a shitty cop-out.
Even in Rugrats, it was obvious that she was just acting as a brat because she was taught it was normal and okay. But since she was an only child and was always away from her parents, she used Cynthia as a surrogate friend to pretend that she wasn't as alone as she felt she was.
And then she tries to boss the other kids around because she felt that she knew better than they did and she was basically emulating her parents who she still thought were the most successful people she could aspire to be.>Their work was important, that's why they're too busy to play with her, it's not their fault, etc.
She always reminded me of the cheesy orphans from other movies who say their parents didn't abandon them, they were international spies on super secret missions, they'll come back when they're done blahblahblah.