Because stupidity affects competence. Race, religion, gender, age (with some exceptions, though even in physically demanding professions it's often the case that experience compensates for physical deterioration), and sexual orientation don't.

And while it's possible to make the argument that becoming pregnant and taking time off to care for a new baby affect competence, there's no getting around the fact that in a fair society, that burden (excepting the actual biological business of pregnancy) would be shared fairly between both parents. Someone has to take care of that baby - it's in all our interests, in fact, that it be well cared for - and it's only because our society still places the bulk of that burden of care on the mother's shoulders that her choice to have a baby affects her ability to work as dramatically as it does. It's wrong to penalize her yet again by discriminating against her when she interviews for a job.