A few things in play here. Each job has certain quotas they have to fill. Too many of one job will make advancement nearly impossible which prevents people from moving up into leadership positions and will eventually cause a shortage when those that can’t promote decide to leave. Recruiters have quotas too, like 2.5 recruits this month or at least one female this month, etc.
Your ASVAB score and likelihood of qualifying for a clearance are also considered. You can join the military without being a citizen (and can get expedited citizenship), but your job prospects will be limited to non clearance jobs, for example.
When you go to the Military Entrance Processing Stations (MEPS), you are put through a bunch of medical tests and once you are cleared for entry, you go on to negotiate the job you want. This is where you have to stick up for yourself. The recruiters there, who you don’t know, will try to push you one way. For instance, I wanted IT which I was told was full. Instead, I was offered “deck seamanship” which I was told let me get a feel of different jobs which I could the “strike” (apply) for. It sounded good, but I really wanted IT. I stood my ground and said I wasn’t in a rush and I could wait until IT opened up. They aren’t letting you out that door if they can help it, and I magically got IT with a signing bonus no less.
Come to find out deck seamanship meant I would be chipping paint and repainting the ship and other really hard grunt work. By the time I would have struck for another job, my peers would have been promoted years ahead of me. It helps to talk to a vet or have them go with you when talking to the recruiters. The recruiters don’t always lie, but they do sometimes only reveal the information they think is important.