They don't believe there isn't some backchannel for the regular government (the executive mostly) to influence a court ruling when it really needs to.
They’re dictators. But no man rules alone. Irrespective of how well they understand the situation, they are constrained by their sources of political power. There is no evidence those sources understand the rule of law or separation of powers.
This is a common failure mode for authoritarian regimes, by the way. A competent leader forced to make incompetent decisions by an elite corroded by years of corrupt rule.
And, in truth, as we've learned with Trump, in many cases they do because it's harder to undo or punish his actions as fast as he can keep doing things. When there are no clear rules for enforcing the rules, we lose.
This doesn't strike me as a reasonable generalization. It implies too much stupidity. I think Mr Putin and Mr Xi are well educated and have a grasp of how the world works well beyond what we lay people think we know.