Because he hasn't earned that value.
I referenced the Matt Schaub trade as a benchmark earlier, since Schaub looks like the best parallel we have (25 year old backup, limited game experience, similar draft position) and ran the numbers through the Chase Stuart draft value calculator. Atlanta received a draft value somewhere in the neighborhood of 19.5 for Schaub, which equates to... the 11th overall pick.
So unless you believe Schaub had substantially more value at the time then it seems NFL teams have been willing to pay a price of roughly the 12th overall for a player similar to Jimmy G.
That was some time ago, so maybe there's some reason that this situation is considerably different. But at a glance it seems like a good reference point.