I recently came across a job listing that lists "5+ years professional experience [in the field]" as one of the requirements but under seniority level, it says "Entry level." What should one take away from this? I was under the impression that an entry level job was a job for fresh graduates or people transitioning from a different field but here, that's clearly not the case. Can it be used to refer to any job that is the lowest level within a team, i.e., "not senior?"
They've either got exceptionally high standards, or (much more likely) the person writing the job ad has no idea about the position they're recruiting for. Perhaps even more likely again is that there was a field that the person placing the advertisement didn't notice or read properly, and "entry level" was the default.
I'd look at the other requirements, and see if they describe a job you'd like to apply for. If they do, then apply, and you'll be able to discover what you need to know during the recruitment process.
If it's on every job listing, then like others have pointed out, it's probably a default field that was missed or a bug on the website.
However, entry level usually means a person just about meets the skills needed for entry, but not enough to work autonomously (without a lot of assistance from colleagues).
I was under the impression that an entry level job was a job for fresh graduates or people transitioning from a different field
Not necessarily. Years of experience doesn't translate to skills. You could have 10 years experience in the field, but if you've only ever used the basics, your skills can still be at junior level and you would only be able to get entry level positions.
It means that you need to ask about the salary expectations that they have. Many places want to get someone with 25 years of experience, but pay for someone with only 1 year. If you expect to get paid for your experience, you need to ask for that and be prepared to walk when they do not match or even come close to what you need.
They’re getting too many applicants.
You’re asking why this is done, so I’ll tell you the simplest reason: they’re getting too many applications, so they then proceed to raise the requirements to reduce the number of viable applications they receive; if they’re getting a thousand applications and 90% of them have less than 5 years experience, then they can list five years experience as a requirement so that they can toss 900 resumes in the garbage bin. The same thing has happened to university degrees; they used to be a lot rarer, so they were listed as required, and now everybody has one so they’ve become functionally mandatory.
Also, in IT specifically, you’ve got a similar phenomenon where companies don’t want to spend time and money training employees who might leave to work for someone else six months later, so they refuse to hire anyone who isn’t “ready to hit the ground running”.