The main reason is that both Democrats simply inherited a deal for withdrawal before a Democrat became President. For the Afghanistan war in particular, around 80% of troops had already withdrawn by the time Biden was inaugurated. Republicans and Democrats seem to get this confused -- many of the issues with withdrawals were due to Republican predecessors negotiating the deal (especially in regard to Trump), and the successor rarely had the power to significantly change the terms of those withdrawals. In Biden's case, he really did have no choice -- returning troops to Afghanistan at the original number would have been tantamount to re-invasion.

I don't really see a difference in parties when it comes to war. Entering/leaving wars seems to be more motivated by approval numbers rather than some party policy. In the meantime, both sides have been fine with creating their war crime detention facilities or drone striking civilians when we aren't paying attention.