Why can the AI use starting pitchers out of the pen, but I can't?
Who did they use?
There are some starters that also have reliever cards. Nolan Ryan, Vida Blue, David Price for example
Jeff Sharkmarjidza (I should note this is stage 2, which already has it's own flaws)
unless he is the first pitcher that you face in the moment, meaning he was the "starter" for the unseen 6 or 7 previous innings, you should not see another starter.