I think they're assuming you will quit in three or four months because you keep quitting or getting fired in that amount of time. Not to mention that with a college degree you're overqualified for the jobs you apply for. They don't want to waste money training you when they can just hire someone else. I guess you could try promising you intend to stay on the job long term in your cover letter.
Maybe find a friend who can recommend you for a job that actually wants someone with a college degree.