Common sense seems to dictate such.
I would be wary of making assumptions based on what common sense would seem to dictate. Common sense is often wrong.
Ok, let's view it from economic reasoning. If T3/T4 schools did not provide a valuable service to someone, they would eventually go out of business. Since most are not government supported, they must be providing a benefit to someone to remain in business. If they served no purpose or were completely ripping students off, they would get a reputation for such and eventually be driven out of business. The fact that they remain in existence shows that they are serving a purpose. In fact, if you research attorney profiles at small firms outside of big markets, I believe that you will find that most of their new associates are graduates of T3/T4 law schools. So, how can the job prospects be so bad for such graduates?
The just don't get the biglaw jobs (in general). So? Biglaw is only one facet of legal work. Many attorneys would find that life repugnant. Others thrive on it.
I think the qualifier that should be added to the assertion that job prospects are bad for non t14 graduates is that this is only true for saturated markets like NYC, DC, SF, LA, ect. Take NYC for example. There are 4 top 30 law schools in NYC. Add to this fact that practically everyone one on earth is competing for jobs at big NYC firms, it's simply no surprise that job prospects in this market are stilted towards the graduates of top firms. Move to Las Vegas or Phoenix and the big law graduates have little or no advantage. In fact, Phoenix and Las Vegas firms prefer to hire locals because of the climate extremes. Alot of new hires can't take the heat in the summer, so they move after a few years. It's all about markets, not so much the prestige of a school. In national markets, national schools fair better. In local markets, local schools compete well.