Can someone explain positional value?

[beach tribe]

I know that FB is definitely no where near the top, or even 2nd tier as far as positional value, but I still don't understand why players such as L. Neal, or T Rich were so overlooked. These guys contributed as much, if not more than most Gs in the league. T Rich was invaluable on running plays. Like I said, I'm not advocating them being more important than any other position, but they are cheap, and effective, why the hell do teams let these guys walk all the time. If I were the Titans, L. Neal would have retired in TN. It's not like a team can't afford to keep these guys.