Why don't universities reveal the extra time they give to students with disabilities when taking exams?
You already gave the actual reason: to hide it from employers. Why? Because it is irrelevant. 

Diplomas/grades from university are the same, for all students. Period. It does not matter how you acquired it, the university recognised you meet the criteria (skills, know-how, ...) for having this diploma. That's all the employer needs to know. 

Edit from comments: 

The employer just has to know the grades/diplomas are assumed to be given objectively.

Time is irrelevant, because extra time allowance is here to level the student impairments to give him a fair chance at proving his/her skill on the subject. Time is usually a factor because there has to be a point where it's finished. The duration is not a factor of evaluation. Diplomas and grades are the same for all students because they evaluate the skills/knowledge of the students and not his/her performance.

If an employer needs someone who can do things in some timely manner, they are the one who needs to test candidates on that criteria. Universities just don't do this.