Mormons believe all of humanity pre-existed this life on earth. At this point you are probably wondering why you don’t remember it. Orson Pratt can explain it.
Orson Pratt was one of the original Mormon Twelve Apostles ordained by Joseph Smith in 1835. Though he apostatized in 1842 he was re-baptized in 1843 and reinstated to the Council of the Twelve Apostles. He died in 1881. He made the following Statement that has not been changed in Mormon understanding since 1854.
“When Jesus was born into our world, his previous knowledge was taken from him; this was occasioned by His spiritual body being compressed into a smaller volume than it originally occupied, … when this spirit was compressed, so as to be wholly enclosed in an infant tabernacle, it had a tendency to suspend memory; …So it is with man. When he enters a body of flesh, his spirit is so compressed and contracted in infancy that he forgets his former existence…” (The Seer, February 1853, 1854, p. 21).
When you are in the business of inventing a religion absurdities such as the above will inevitably occur.