The short answer is they don't.
  They scan your resume with a very specific set of data in their mind and if they determine your resume matches this data only then will they decide to actually read your resume.

I've personally reviewed probably at least a million resumes in my 40 years in this business.
  When you review 50 to 200 resumes per day, they add up and at this volume, it would be impossible to "read" each of them and it isn't necessary for me to do so.

If you're a candidate, you may still get angry about that, but at least I'm going to explain the reality of what happens with your resume in this article to help understand why it's done this way.
I know that candidates often get angry about the way their resume is quickly dismissed, but they are often to blame for not preparing their resume to be the best it can be.
Check your resume against my top 9 "knock out" criteria below to see how your resume stacks up.
The Recruiter Scan Process Revealed Here's how I process the resumes I review every day and the criteria I use to move some to the "no interest" group and the others into "follow up.
" I set up a hierarchy of certain "must haves" or you're out, so at first I'm really just box checking.
These must haves are what the job spec calls for.
If you're reading a job posting and your resume doesn't match the stated criteria - and it isn't easily and quickly apparent - then goodbye.
Generally, 80% of the time these must haves are my knock out blows.
There are exceptions to each of these, but I'm dealing with the 80/20 rule.
This is box checking, if I see any one of these as I scan your resume you will be excluded.
The 9 Critical Resume Knockout Blows Location.
If you're not local to the job, I immediately know the process takes on an added cost to my client and becomes much more complex.
This includes the physical move, housing, temporary lodging, possible spousal employment, and schools to name a few.
All things being equal, clients will opt for local candidates.
I don't make this a knock out factor right away, but neither does it fall into the plus column.
This takes about 1 second.
Functional resume.
The most common reason for a functional resume is to hide something and I know that all too well.
It raises a red flag and I rarely pay much attention at all to a functional resume at first and just quickly skip to the lower third of the resume to glance at your work history.
  Unless I see something compelling, you're gone.
Another big, big disadvantage is what I call "trend over time", i.
e.
trying to get a sense of your job progression.
  Are you moving steadily upward, or have you hit a plateau or have you actually gone backwards.
  This is impossible to tell from a functional resume because titles can't adequately tell your story.
About 5 seconds.
Top one-third.
I review the information just after your contact data.
Don't bother with putting an objective on your resume.
  I'm not looking to see what you want to do, I'm looking to see if you fit my open jobs.
Don't fill this valuable real estate with soft skills either.
Give me your best elevator pitch in a Summary section and make sure it maps/ties to the job.
This more than anything else will slow me down and start changing me from scanning to reading.
  If you don't have a concise summary then I usually just skip anything else written in any other format, at least on the first pass (and likely my first and only pass through your resume).
About 10-15 seconds to give me a reason to slow down and read.
Job stability.
You hear a lot about "no more gold watches" and no more loyalty and therefore job stability doesn't matter.
  Wrong!  It doesn't matter as much as it use to, but it still matters.
  If you've changed jobs every 2-3 years, why would my client think you're going to stay with them any longer?  And at a hiring cost that can be one-and-a-half times your base salary (150%), they do care and so do I.
  About 10 seconds tops.
Level.
If you've been at a VP and above level and now applying to a Manager or Director level, I get very cautious very quickly.
I next try to determine the size of your employer and the scope of your role.
  If you've been a VP at a larger Fortune employer, and now want to be a Manager at that same size or smaller company (and if you're now unemployed especially) I will move on.
Takes about 5-10 seconds.
Missing data.
If you've left off your earlier job history, that can be a big red flag.
  When I get to the end of your job history and see a job title that is inconsistent with the level of job where you would have started your career, I know right away you're hiding something.
  While you may not provide as much detail about your earliest employment, I really have to be convinced by everything else before I'll bother to take time to talk with you.
About 5 seconds.
Role.
If you've been doing a job that has little or no relevancy to the job you applied for, you're gone in less than a few heart beats.
If I'm looking for sales, you'd better have had sales experience.
Recruiters like me are paid to find specific experience and I know better than to try to provide my clients with anything less than what they've asked for.
About 5 seconds to make this determination.
Industry.
This isn't always critical, but often it is.
If your experience has all been in banking and the opportunity I'm recruiting for happens to be in aerospace AND requires specific knowledge that would only come from being employed in the aerospace industry, don't send your resume.
  I'll spot it in about 5 - 10 seconds and your resume moves to the "not a match" category.
Education.
Like it or not, most employers have defined their optimum and minimum educational standards.
While I often disagree with them, I know they're firmly set on this criteria and it's my job to give it to them.
  This can sometimes include certain certifications as well.
I can determine that in about 5 seconds and if you don't have them, you go into the "not a match" group.
After all this, 80 - 100% of the resumes have been eliminated.
If there are any left, then I will take the time to actually read them in detail.