I'm only talking about the bench press from a powerlifting perspective, so I gave examples of different bench pressers with different pressing techniques. It's true what you say, that the front deltoids and triceps do most of the work, but for some reason, it's the pectorals that tear in this exercise.
My interpretation is that in a powerlifting bench setup, the scapulas are retracted, depressed and actively pinned to the bench via thoracic extension (and lat tension obviously), so with the same bar path, the strain is shifted almost entirely onto the pecs compared to a traditional barbell bench, where the shoulder blades slide along the ribcage as the weight moves.
This setup also places the pec origin lower and more posterior (more stretch)
Another crucial detail: during a bench, powerlifters externally rotate their humeruses, which rolls the pecs outward (retarded term, I know) and increases stretch.

Look at the elbows

With all of this in mind, it’s not surprising that pec tears can occur during a powerlifting style bench press.
