TCP also uses CUBIC. So, you would have just as well already been able to be a dick by configuring your CUBIC in TCP to be more aggressive.
Lots of research has gone into an optimal configuration of CUBIC for TCP, under the presumption that everyone will use this implementation, and therefore with the goal of optimal results for everyone.
QUIC now falls out of line because early adopters get better results, as they can unfairly compete against the fair TCP implementation. In the long term however, if QUIC is adopted by everyone, this will lead to a worse result for everyone, because this optimal configuration that's been researched under TCP has been thrown out for no good reason.
At least I'm not aware of a good reason. Maybe QUIC is actually somewhat less impacted by packets being dropped or something like that and does work better with everyone just trying to squeeze everything through aggressively, but even if that is the case, I cannot imagine it being the case to the degree that this CUBIC configuration has been made more aggressive.