Most Mormons are Mormon not because the church is true, but because their parents were Mormon.
If they had been raised Muslim, they would’ve prayed to Allah and discovered that Islam is true. If they had been raised Jewish, they would’ve prayed to Jehovah and discovered that Judaism is true. If they had been raised Catholic, they would’ve prayed to God and discovered that Catholicism is true. If they had been raised atheist, they would’ve prayed to no one and discovered that atheism is true.
Ask any person of faith what their religion is and what their parents’ religion is, and there’s a super good chance they’re the same.
Certainly, Mormons will say they prayed to know if the church is true and have received a spiritual witness that it is true. However, if you ask them which other churches they prayed about to know if they are true, it’s likely they’ll say none. It’s understandable that if the only church they pray about is the LDS church that the spiritual experience they have confirms what they’ve already been influenced to believe.
