Because he didn't divebomb him. Piastri was squeezed so hard that he not only went off the track, but also made contact with the wall. You can't get squeezed into the wall if you divebomb someone. That's not how divebombing works
Piastri was far enough along to be entitled to space, and ignoring the fact that Sainz locked up, he could have given him more space.
To be fair, i consider it a racing incident. It was Lap 1, and Sainz, despite the margin and the lockup, was focusing on avoiding Hamilton on the outside. But definitely not Piastris fault.