About 25-30% of the retail value of a carcass is in the neck, shoulders and chest.
If there's any damage, the game dealer / butcher has the labour cost of trimming and tidying it up. If it's badly damaged, it's not worth the effort, so the whole front end will go in the bin.
Why should they pay the stalker a premium price for something that's going in the bin? Answer: they don't. That's why the price is so low. It's to allow for the wastage.
And then there's the added fact that waste disposal needs to be paid for, along with business rates, staffing costs, etc.
As for the pet food market, I suggest you read up on the guidelines for the production and manufacture of pet foods. It's not as simple as diverting unfit meat from the human food chain to the pet food market.
If they were to only take head shot deer then the price would be higher, but stalkers expect game dealers to take their rubbish as well. Who's shafting who here?
And when a game dealer does offer higher prices for head shot deer all the (less competent) stalkers moan that they're encouraging unethical shooting.