When was the last time that a super bowl winning quarterback under the age of 30 hit free agency?
No offense, but are you even paying attention? Quarterback is the most sought after position in the NFL. Middling players who have hardly any starts or stats to their name get big contracts. Mike Glennon got $18 million for playing one year with the Bears and starting 4 games. Jimmy Garoppolo has 7 career starts and just signed a contract with $70 million guaranteed. Cousins is 29 years old, and for the past 3 years has averaged over 4000 yards and 25 TDs per season on a team with average, at best, offensive talent surrounding him.
Guys like that very rarely hit the open market. When they do, there will be a bidding war. It's that simple.