The Black Mercy doesn't really give you your heart's desire. That was a misconception on the part of Mongul
Are we supposed to believe that Superman's heart desire is Krypton surviving... and becoming a dystopia in which his mother is dead, his father is crazy, his cousin has been brutally beaten, his family is being targetted by lunatics, his dog Krypto is non-existent and Kandor has been reduced by Brainiac? And Batman's heart desire is getting married to Kathy Kane instead of one of his main love interests? No. What the Black Mercy does is showing you a vision of something you despairingly desire (such like Krypton not blowing up, or Thomas and Martha Wayne not dying) so you want to lose yourself in the fantasy, and then twisting it so you are trapped in a nightmare.- Mongul's fantasy seems flawless for him though. Of course its possible the Black Mercy can only set up your heart's desire and cracks end up showing up, and eventually his rule because a Pyrrhic Victory.
The Black Mercy gives you a simplistic "perfect fantasy", which is more than enough for people who never question and who accept whatever is the most pleasing to believe
As Mongul's fantasy shows, a shallow person will love the Black Mercy's idealized fantasy world. But Superman is not a shallow person, and he will not lie to himself even when it would hurt less for him to do so. So his mind naturally extrapolates from the simple "perfect fantasy" what it would really be like, showing a depth that Mongul could never imagine that Superman (or anyone else) might have.