How about because he, Satan is a deceiver? Who mixes truth with falsehoods.....
What better way to deceive? Satan knows the bible better than most humans...God knows..His ex-head praise and worship angel knows who His Son is. Lying & deception is his mark.
Sincere prayer or not....the one that roars like a lion, is a liar. And you wouldn't be the first one to be fooled.
Just because you prayed, and think you got an answer doesn't mean it was from God. I've known too many folks that have been tripped up like that.
The exact opposite of what you have said could be said of many who've left Mormonism...They grew up in the LDS way...and had revelations/questions/prayers answered that "proved to them" that Mormonism was absolutely false. How is that?
Like I’ve said repeatedly, if God allowed me to be deceived by Satan when I went to him in sincere prayer; he then is not worthy of my worship and I will accuse him of lying to me at Judgment Day. I’ll gladly go to hell than worship such a capricious god. He is untrustworthy and subjective.