Well, let's see him play more outstanding football and earn his next contract. That benefit the Hawks now. Even 5-6 year deals are being renegotiated. What's to say DK won't prefer to extend in 2 years if things are good and doesn't want to risk just one more season?
That's the beauty of those 6 yr deals. In a few years, you're through the guaranteed money part, if he's still performing he's now a bargain. If he were to fall off a cliff for whatever reason, he can be cut/traded for very little cap ramifications. It's win-win for the org.
Instead DK's contract is nearly fully guaranteed he get's to make bank, and then go back to the negotiation table again in 3 yrs (with 1 year left on his new deal.) And ask for $35-40M APY as the salary cap, and thus salaries continue to increase.
Overall, it's still a good deal for the Seahawks, it's just these shorter deals with their superstars cost them so much damn money against their cap in the longrun it's ridiculous.
For example: Wags 4 yrs $40M extension made him the highest paid MLB in football at the time. Had it been, 6 yrs $60M. They would've avoided ever having to pay him $18M APY. These short 2nd contracts are killer.
Another: Lockett: 3 yrs 10M APY, now he's on 4 yrs $17.5M APY.
Imagine they had signed Lockett to 6 yrs $12M APY originally. Much better.
This is how the other teams in the league operate to maximize their cap space, and maintain flexibility in the later years of these contracts when the guarantees run out. It's not some magic trick, every team does it. Except the Seahawks.