Because going into a Guard or Reserve Unit is highly competitive. Only 2-4% of people looking to get hired into the Guard or Reserves get pilot slots. That means you should have some substantial time (200 hours +) if you want to be really competitive. Not as true for Active Duty.

In the Guard you would go to AMS (Academy of Military Science), which is a 6 week version of OTS. In the Reserves you will go to to OTS (Officer Training School), which is a 12 week program. You are an Officer first, so if you didn't go to the Academy or ROTC in College, then you are going to go to AMS or OTS. Why? Because whether you are Guard, Reserve, or Active duty, you are an Officer in the United States Air Force, and are trained accordingly.