Why don't universities reveal the extra time they give to students with disabilities when taking exams? Many (most?) universities have a system of allowances for students with particular medical conditions/learning difficulties including dyslexia, dyspraxia and others.  The most common allowance is extra time in exams.  Sometimes this can be an extra half or whole hour but in other cases can be, for example, 8 hours allowed for a 3 hour exam.

There are various arguments in favor of giving this extra time but of course the merits of doing so can also be debated.

When the student graduates, as far as I know no universities include a record of this extra time on their official transcript.  This appears to hide an important piece of information from a potential employer and also makes it very hard to compare one set of grades with another or to interpret the significance of any particular set of grades.  Of course an employer might not care how long it takes a student to solve a problem or write an essay as as long as they do it well. But for some employers it might make a crucial difference.

Why don't universities provide this information on the official grade transcript that a student gets?
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.