TL;DR: There are many reasons why you may not hear back from an employer after applying for a job opening. Sometimes it's them; not you. Other times, it's your job search strategy and collateral. Rejection happens. Radio silence is common. Don't get caught up in the emotion of it when you're not even sure you're the problem.
1. It's not the company's practice to reach out to candidates they're not interested in.
After submitting a job application, you're expecting a response. Whether it's a software generated response or a personalized email from a recruiter, the expectation is that you hear something. Unfortunately, every employer does not respond to every candidate because of the volume of applications they receive daily. For example, you may receive an email like this:
Thank you for your interest. We received your application and will be in touch if your qualifications match our needs for the role. Due to the volume of applications, we want to be respectful of your time and let you know that we will only be reaching out to candidates who are an immediate fit for the role.
2. You're not qualified for the job.
Just because you're interested in a role, doesn't mean your qualified for it. Applying for roles you're not qualified for, will debunk your application for the applicant tracking system, causing you to receive a rejection email or no email at all. Applying for roles that you're qualified for will increase your chances of hearing back from an employer.
3. You submitted an incomplete job application.
Are you sure the recruiter received your application? Using job boards to aid your job search is smart. Only applying for a job through those job boards however, is not. Applying for a job through Indeed or LinkedIn's Easy Apply for example, doesn't transfer your application over to the applicant tracking system, which means that the recruiter may have never received it. Applying directly through the company's careers page will ensure the recruiter is able to find your application.
4. Your resume and cover letter wasn't applicant tracking system (ATS) ready.
If you haven't taken a deep look at the job description to see exactly what the employer is looking for, your generic resume and cover letter may not have spoken to the job description in a way that illustrated your qualification and fit for the job, using the keywords that the ATS was looking for. If the ATS doesn't give your resume a high rating, it's likely your application will get rejected once a recruiter has found a solid group of candidates that rated higher.
5. They're further in the interview process and hired someone before removing the posting.
When you find a job posting, you don't necessarily know how long that job has been posted or where an employer is in the interview process. You may have found it weeks or even months after it was posted and the employer could have already started the interview process with a few qualified candidates. If you don't hear back after applying, they may have already identified a candidate they want to hire and just haven't removed the job posting as yet. In this case, it's them; not you.