No one in their sane mind want to drive on Market down there these days anyways.
But you have to cross market to allow the city to function, and there's a few major intersections, especially 7th,8th,9th, and 10th. This is pretty much the only way to get access to the south end of SoMA and I-80. You can't "escape" via the Mission, the last option is 13th which exits onto Market street at Duboce triangle.
You can't go under, Bart/muni is there. Only other option is to go over Central FWY style, and the city is more in favor of tearing out freeways.