Because the elder McMichael knew exactly who the 'jogger' was, and had every reason to suspect that the 'jogger' was armed. The 'jogger' had NO WAY OUT. He knew if the McMichael's detained him until police arrived, he was a three strike felon who would be going to prison.
If the owner of the house under construction didn’t press charges, then Arbery would have been released as he technically didn’t commit a crime.