The Germans spent a large amount of manpower in adverse territory for this city, even after long exhausting continuous combat.
After the war, Kliest wrote:
The capture of Stalingrad was subsidiary to the main aim. It was only of importance as a convenient place, in the bottleneck between Don and the Volga, where we could block an attack on our flank by Russian forces coming from the East. At the start, Stalingrad was no more than a name on the map to us.
Yes it is a fact that Stalingrad was not significantly resourceful to fight for. Why then did the Germans, after seeing that it was to be a long affair, not just encircle the city? Instead they kept fighting for it in close quarter combat for the city's buildings, which is almost always a disadvantageous, slowing and depleting affair for an invading force. Considering the momentum of their initial advance, could they have fared better by crossing the Volga, and laying siege to city?