A better question might be:
Why do people NOT reveal their "real selves" early on?
In the beginning of most (new relationships) there's an "infatuation phase" whereby both people oftentimes bend over backwards to impress one another. The word "no" is seldom if ever used. No one wants to say/do anything that might risk them "blowing it" with the object of their affection. Conversations flow for hours, laughter comes easily, cards, token gifts, and weekend getaways are done "just because" and sex is off the charts! They make (each other's happiness) their top priority.
Everything (you) want to do (they) want to do!
Each of you believes you've met your "soul-mate"!
It's not until there is an "emotional investment" or commitment established that most people feel it's (safe) to reveal their "authentic selves" without the risk of possibly losing their mate.
After the first major disagreement your "differences", "boundaries", and "deal breakers" become more apparent. The word "no" is no longer a foreign word in your relationship.
It's almost a cliché to hear someone say:
"He/she is not the same person I fell in love with!"
Had you revealed your "real self" early on they never would have invested their time and emotion into the relationship. They would have known that you were not the "right person" for them and they weren't the "right person" for you.
Where as if both people applied the "be yourself" philosophy from the beginning they could have avoided getting involved in the first place. Early rejection saves everyone time.