Because offspring with two parents will share half of each parent's DNA.
Explanation:
Sex cells contain half of the genetic information of an organism's regular cells. This is because a sexually-produced organism will be unique; it will share genetic information with both of its parents, rather than be identical to its bearer (like an asexual organism would be). When a sex cell meets another sex cell, their DNA will meet as well and change/adapt to suit the organism. If a sex cell had all the information needed to create an embyro, instead of half, which requires another cell's information to fill the DNA void.... well, it would just do it.
