Because false positives would cause dangerous accidents as well.

And worse (heavy cynism ahead): they would make the car look stupid even when not causing an accident, whereas boldly plowing into whatever might or might not be an obstacle makes the car look strong and confident, right until it looks all bent and crumbled. Car sales are incredibly marketing driven and a self-driving implementation that routinely chickens out of ambiguities that look easily solvable to humans would be a worse marketing problem than twice the number of accident investigations Tesla is facing.