Same reason a still very good Tra Thomas was only getting a few offers...he's old, and its not certain how much longer he has left in him, and he wanted a sizable deal (if $5M/yr is still sizable).
They still played well last year...but there are questions as to how much is really left.
I'd guess that barring injury, guys like Pace and Thomas will be above average tackles for the Bears and Jaguars at worst...when you are as talented as these guys, only injuries beyond recognition can keep them from being less then serviceable. Offensive linemen can last a long time in the league if the talent is there. Look at guys like Bruce Matthews and Kevin Mawae or even tackles like Wayne Gandy and Willie Anderson. You can still play even after your prime in the NFL as OL.