They could have placed the funnel anywhere else in the illustration, but they chose not to.
Most people’s rectums angle slightly to the right. Please see a doctor if you have an atypical rectum.
It’s probably the best place for funnel insertion at the end of the day. A hole left behind there is appropriate, you don’t want to mess with any details you might get from the face, and it’s way funnier this way.
The best place would probably be the first place you found. The archeologist wouldn’t have any clue to the exact position the person was in and over digging could ruin the find
The person making the illustration though knew what they were doing.
I do wonder how they knew it was a person shaped cavity vs a plain old hole in the ground.