I'm not sure "speed to siege" really fits the bill. The Germans were still trying to launch major offensives like the Kursk attack into mid-1943, well after Stalingrad.
Limited manpower and equipment - Sixth Army didn't really have the tanks, mechanized infantry, or logistical suppor to launch two independent pincers around Stalingrad - dividing an already weak force would have been inviting defeat in detail by Soviet counterattacks. Going straight for the city let Sixth Army better concentrate its force and make the most out of its logistical and fire support.
Road and transportation networks - The Soviet transport system was pretty rough in places. While sweeping pincers were possible when there were flanking roads or open terrain, sometimes major road and rail hubs had to be taken in order to allow for further campaigning to be supplied.
Geography and Soviet responses - In the cases of Sevastopol, Brest, and other coastal cities, the town couldn't be fully enveloped by Axis troops. Leaving them lightly-guarded raised the risk of a Soviet breakout or attack, so reducing the city was often a safer long-term option for the Germans. Leaving a large group of Soviet troops in a city where they could be supplied by sea and potentially build up strength (or be evacuated) wasn't ideal. For Stalingrad, its position on the Volga meant encircling troops would have had to make and hold two precarious bridgeheads with Soviet troops still on both banks of the Volga. That was hardly an appealing prospect. It was doable (the Soviets did it during Operation Uranusb albeit with more men), but very, very risky.