There are two reasons. But before I list them, you must understand that the topic your question addresses is the POSITION of a person. Position, meaning, his legal standing. To illustrate, imagine that a country was divided right down the middle, with one side called "Sin" and the other side called "Righteous." You are born standing on the side called Sin. This must be stressed because there is a big difference between positional sinfulness, which all unsaved humans equally share, and actual sinning, which all human have in varying degrees. Now the reasons that positional sinfulness is what it is are these.
1. Adam was the representative head of the human race.
Adam was the father of the human race, and thus all humans would descend from him. Therefore, when God pronounced His judgment on Adam, He was in effect pronouncing it upon the entire human race. Adam's sin actually ushered sin into the world, that is, all of nature, and his descendants will necessarily be born into that nature.
2. Adam was a type of Christ.
When we say Adam was a type of Christ, what we mean can be stated in a few different was. For example, The work of Adam was emblematic of Christ's work. Adam's work was a foreshadowing of Christ's work. The point is, that it was God's plan from the beginning that Adam would be a reverse symbol of the salvation brought by Christ.
Just as Adam positionally represented all of humanity in the Garden, so Christ positionally represented all of humanity on the Cross. And just as Adam's sin ushered in death, so Christ's death ushered in salvation.
"For as by one man's disobedience [Adam's sin] many were made sinners, so by the obedience of one [Christ's righteousness] shall many be made righteous."
Primary biblical texts: Romans 5:12-19, 1 Corinthians 15:21-22.
