September call-ups don’t work the way they used to. Teams used to be able to call up their entire 40 man roster if they wanted to. Some teams called up a couple players, some called up 10 or more.
That is no longer the case. Starting September 1, every team must maintain a 28 man roster with a max of 14 pitchers. Those extra 2 players can be swapped with other minor league guys over the course of the month, but you no longer have endless options on the bench or in the pen.
If you want to give a bunch of rookie playing time to show what they’ve got, you have to do what the Angels did and DFA multiple veterans.