Different people have different needs. I own 3 pair of bike shorts, all about $25 or so, really cheap, and have been switching between them for 5 years and 21000 miles now. They work fine for me. I ride 21 miles every day, and once in a while I do 75 miles or so.

Jeans are only good for a couple of miles, after that, they're rubbing on things that you don't really want rubbed that much or that abrasively.