Because the Green Bay Packers are already paying a lot for the ones they've got. From Tom Pelissero at the Press-Gazette:
Nick Barnett is due $4.55 million in salary and bonuses. Clay Matthews is due $1.96 million. Brandon Chillar, who doesn't start in the base defense, is due $1.9 million in base salary alone. And Brady Poppinga, another backup, is due $2.4 million. Toss in Hawk, and that's already well over $15 million in five players -- without a clear starter at left outside linebacker, a position the Packers might fill with a high draft pick.
Throw in the possibility of resigning LB Aaron Kampman and it grows even higher. They need depth at linebacker in a 3-4 defense, but they can't spend their entire budget on linebackers.
Speculation from Bleacher Report about LB Antonio Pierce as a Packer is ridiculous. The Packers don't need an old linebacker with declining skills against the run and in pass coverage to play middle linebacker. They already have four better middle linebackers on the roster.
Speculation from Pro Football Talk about LB Joey Porter is crazy too. GM Ted Thompson doesn't pay top dollar for 33 year old free agents anyway, but Porter has struggled in 2 of the last 3 seasons, and he's become a major off-field distraction.