You'd be surprised how ineffective fear can be as a mechanic. Look at it this way. When your liege imprisons or executes you in this game, do you usually react with cowed submission, or do you start plotting to take him down? Having said that, why should your vassals be any different? Most depictions of cackling medieval tyranny come from pop culture and movies, and those that don't are either propaganda or often known to us precisely because the ruler in question became so notoriously bad at ruling he got overthrown.
There's a mechanical reason as well. Revoking and assassinating and arbitrarily imprisoning all give the ruler significant freedom of action. If doing all these makes your vassals more likely to obey you, in addition to their direct benefits, the game risks just becoming far too easily. People have complained that there is no real reason to execute prisoners except for RP reasons at present and, while that's true, adding a "fear factor" might just create the opposite situation.