Either you don't have the security deposit... thats a problem, or you don't have enough income... bigger problem, or your credit is shot.
Not necessarily. The ad says that s/he is a college student. I live in a college town, and most apartment complexes won't rent to students without a cosigner. So this person could legitimately be responsible, have the money, etc., but be required to have a cosigner because of age & student status. If there's no family member who can do it, they're shit out of luck.
I'm not saying I'd cosign for some kid I didn't know (I doubt I'd cosign for a kid I did know, unless it was my kid), but this isn't necessarily a case of them being irresponsible.
Assuming the student is over 18 and a legal adult who
can sign a contract, the student status isn't the problem. If the student were independently wealthy with a passive income four times bigger than their monthly rent, they'd have no trouble getting approved. It's a question of income. If the student doesn't have enough income to qualify properly, such that they make enough to ensure rent is less than 30% of their monthly income, then a cosigner is vital. Anything else constitutes predatory lending.
Just because students "have the money" to pay the rent doesn't mean they have enough money to
reliably pay the rent if something goes sour. A whole lot of students live hand-to-mouth, which is fine if you're living in a dorm and your expenses are mostly fixed. Outside the dorm, in the greater community, students have to meet the same standards as anyone else seeking a rental, and that means having an income substantially higher than what's usually associated with student housing.
Hopefully each of these renters has a separate lease or rental agreement, but most landlords have just one lease and require all of the tenants to sign it together. This is very much in the landlord's favor. Unless each tenant's liability is explicitly spelled out, tenants are jointly and severally responsible for the
entire lease, as are their co-signers. This means that if the other two tenants bail out or refuse to pay rent, the co-signer for the single tenant could easily be liable for the entire amount of the lease.