I think the reasoning is that since Jews lacked foreskins but their neighbors the philistines had them, bringing the foreskins showed that it was not Jews you went out to kill. It's like if you wanted someone to kill 100 rattler snakes and as proof you bring 100 rattlers as that's their distinctive body part.

Historically, heads weren’t all that common. Usually only for specific bounties where you’d need to show it was the right person.

For more general things, scalps, ears, and fingers were all far more common.

I mean think about it. It’s gruesome enough on its own. But, fondling dead guy dick is… especially unpleasant. (And they probably stank.)