Why ? You don't explain it in your post.
And I'll tell you why not, imho: Piety represents how well you follow your church's school of thought. Usually, church favors humility and this school of thought opposes the common ways to gain prestige, so it's hard to have both high Prestige and high Piety. But if you do manage to have high Prestige and Piety, you deserve to have the two opinion modifiers.
Let's have a concrete example: if a conqueror king is cruel and manage to keep his internal realm stable with an iron grip and lots of executions, he will have high Prestige and low Piety. If another King manages to conquer as much but keep his realm stable with Diplomacy instead of Tyranny, and manages to keep enough gold to pay homage to priests, he will have the same Prestige but also high Piety, and he deserves the opinion boost.
To get the +20 from Prestige, 2000 prestige is required. To get the +20 from Piety, 500 piety is required. Secondly, as I said, this wouldn't impact most characters, just those with a government type that let them hold both castles and temples without penalty.
Also, it isn't hard
at all to get up to those numbers, without even paying attention to them. Kings can get above both with little difficulty, none if they are fighting wars and Holy Wars, which they would do anyway. Emperors, even AI emperors, can build up a ridiculous amount of both:
This plays a part in the Muslim superblobs in the early start. One is an empire to begin with, and the other almost always forms one, and has the piety from Holy Wars before then.
To be perfectly clear: almost all characters only care about piety or prestige currently, not both. The Pope only cares about your Piety (assuming you are Catholic), and your vassals and fellow rulers only care about your Prestige. However, those that can hold both holding types get access to both bonuses, so where most characters only have access to +20 with any given character, they can get +40 with all other landed characters of their religion.
As a side note, your example is rather nonsensical. Even for those that can get both bonuses, they cap at at +40, but if one is Tyrannically imprisoning and executing characters, that is -50 each, -40 for the imprisonment and -10 for the execution. Ignoring the opinion penalty from imprisonment, that is still -10 for each execution, at a cost of 20 piety per execution. As the bonus opinion from prestige caps out at +20, that is just two executions before you break even, at a cost of 40 Piety. ( Four executions if you get both bonuses). This would just serve to destabilize the realm more, as your remaining vassals would like you less and less, as well as the heirs of the characters you executed, who would have additional negative opinion modifiers. Executing characters to try to stabilize your realm simply isn't viable.
Also, I'm rather confused by what you meant by paying homage to priests.
