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.

College degrees are not a tool for evaluating employment eligibility.

Despite the way it has been used in recent times, college diplomas are a certification of higher education in a particular field - they were not designed to be a measure for employers to look at and evaluate for hiring employees, and they still aren't designed that way.

There is a strong association between having a higher education degree and getting a job - and that is because employers do look at the higher education their prospective employees have.

This doesn't obligate the school to modify their curriculum for those employers. While there is an incentive, because schools that have a high number of graduates find employment in their field of study are far more popular, this is a secondary goal of education, and always has been.

You can argue otherwise if you wish, and some academics may even agree with you, but the point is they don't have to agree with you, and they don't have to disclose additional personal information about a student just because it would be convenient for you. And that includes any 'extra time' they may need to complete course materials.

Since nobody seems to have mentioned it yet,

it is very likely illegal

In the US, at least, several laws (including the Rehabilitation Act of 1973 and the Americans with Disabilities Act) prohibit discrimination on the basis of disability. Section 504 of the Rehabilitation Act also prohibits unauthorized disclosure of a disability or presumption of disability without the express permission of the person in question. It could be argued that marking those who received accommodations for a disability is disclosing their status as disabled, thus violating this law.

And while it may be helpful for an employer to know if a prospective employee is disabled, this is generally only helpful if the employer wishes to discriminate. Why else would they need to know of the disability during the application process? If an accommodation is needed, this can be taken care of after the application process, once the possibility of discrimination has been severely reduced. Of course, one does need to disclose a disability to receive accommodations for it, but only if one wants accommodations. For example, as a disgraphic, I cannot write legibly and type most of my work. If I were working as say, a software engineer, I would not disclose because I need no accommodations (and the disability is therefore irrelevant to my job).

This source explains it far better than I can, but it seems

Under Section 504 and Title II, recipients and public entities may not provide different or separate aid, benefits, or services to individuals with disabilities, or to any class of individuals with disabilities, unless such action is necessary to provide those individuals with aid, benefits, or services that are as effective as those provided to others.
Essentially, because notating use of accommodations is not related to to providing "aids, benefits, or services" to help mitigate the students' disabilities, it is prohibited by law as discrimination on basis of disability.

Incidentally, the College Board and ETA got in trouble for this exact action a few years ago. source 1 source 2 - note that SAT flagging was removed after the article was published

On a further note, many feared this would allow abuse of the accommodations system, but these fears proved unfounded. In fact, many students, including me, were not able to get the accommodations they needed in a timely or even any manner. They've supposedly reformed that this year, but we'll see how that works out...