The adjunct professors are akin to substitutes in the K-12 system. They are there to handle overflow classes, and most do not have a PhD. Most have their Masters and are teaching as an adjunct to get their feet wet and find out if they want to pursue a career in academics. Also, I don't know many that rely on their adjunct teaching to make a living. Most work full-time jobs (as attorneys, health professionals, accountants, and other occupations within their field). There is a vast difference between adjunct and tenure track, and that is why the pay scale is different.
As an aside, adjuncts get paid $1800 per class, per sememster. The Spring semester goes from Jan 7 to May 7 - or 18 weeks. If an adjunct teaches one class for 3 hours, and then has 3 hours of office hours during which they meet with students, prepare their lesson plans, and grade papers, ect... that is 6 hours per week. When you think about it $16.50 per hour isn't bad money for a part-time gig. Oh, and they get a paid week off for Spring Break. Even better.