If they are not going to be the next government of Afghanistan, why does the US need a peace deal with them

A "peace deal" is usually forfeited by the failure of one side or the other to honour the "peace" part.

And the deal was never to supplant the Afghan government with the Taliban. It was just to end the fighting between the Taliban and the coalition.

https://www.cfr.org/backgrounder/us-taliban-peace-deal-agreement-afghanistan-war

If they are not going to be the next government of Afghanistan, why does the US need a peace deal with them

A "peace deal" is usually forfeited by the failure of one side or the other failing to honour the "peace" part.

And the deal was never to supplant the Afghan government with the Taliban. It was just to end the fighting between the Taliban and the coalition.

https://www.cfr.org/backgrounder/us-taliban-peace-deal-agreement-afghanistan-war