Two reasons, I think:
1) My parents don't know much about cars, and having a 2000 Xterra which has intermittent problems here and there, they're incredibly wary of purchasing a used older vehicle.
2) Yes, image. They er.. don't want me appearing unkempt, something about wanting grandchildren some point before I'm 40. I often get berated for dressing in cheap jeans and Fruit of the Loom Ts (my preferred casual ensemble). If it were solely my money, I'd buy something competent and decent looking. But since they're offering to help with payments, I might as well go with the flow.
Also, most of my friends are car guys, with older Japanese cars. They do tend to get problematic, and since I'm no mechanic, I won't be able to tolerate such things as much as said acquaintances.