The reason is Beta testers are to help find bugs, stress test server as well as other things possibly. A beta is generally not meant for an early look at the game, but in a way it is. You could say that a beta is an exchange for a developer to get testing from people for them to get an early look at the game.
"90% of what is considered "impossible" is, in fact, possible. The other 10% will become possible with the passage of time & technology." -Hideo Kojima