It's simple common sense: if most of the people you know that are looking for jobs are not finding them, then why would you take the same failing approach? You should emulate those few that seem to find the job more easily.
Years ago I heard a story about a radio station and how they rented broadcasters. Being a radio personality was a popular ambition at the time. A contact within a radio station told me they had stacks of tires delivered daily. When they had occasionally had a position to fill, they did not bother looking through the resumes – they hired someone they knew.
Today, we have zillions of people submitting their resumes on job boards. Most keep submitting and submitting with dismal results. People submit on average 40 job applications on a job board for every call they get. The people that do get called, not surprisingly, are a 'perfect fit' at least on paper. That means that whatever exact keywords the hiring manager typed were in the resumes.
Matching the Job Posting Exactly
That means you have two choices. You can submit resumes that fit exactly the keywords most likely typed for a position. This method does work, but requires a lot of individual study of the position offered.
The VERY Quick Look
You need to not only match the job posting, to pass a keyword search, but you must also pass the 3-10 second look that the recruiter will give each resume. Put all important information at the top of the first page of your resume. Keep measures of success very visible in bullets.
Connecting to the Person Hiring
Or, you can find a way to know the person hiring. Getting a referral to the hiring manager can result in a 20 times improvement in your likelihood of getting that phone call inviting you to interview.
The LinkedIn Connection
Surprisingly to most people, an applicant is 5 times more likely to be hired for a job based on a search of LinkedIn profiles than if you submit for the job on a job board.
That means your LinkedIn profile should be complete and optimized as well. Since part of the benefit of LinkedIn is the references and endorsements make sure you use these features.
In fields where it's hard to find work, consider also adding work samples in LinkedIn.
Stand Out from the Crowd
Make sure that you can clearly state why you are best for a position. Add an attention getting form of proof if possible. Certificates can help, as can recommendations, but do not limit yourself to the traditional. I've corresponded recently with a guy who printed up what looked like a flyer and used it with his resume as a cover letter. It would not work in all industries but did in his.