That's because you are looking at QBs with older contracts (and they don't have to be that old) as if their contracts were negotiated at the same time and under the same market conditions as those getting $30MM+. Quality QB's signing today, under today's market conditions, are getting $30MM+, and because QB's got $27-28MM a year ago (Garoppolo/Cousins), and $25-26MM 2-3 years ago doesn't change where things sit today.
Markets move on. They don't just stagnate when the best QBs sign a contract and not move until those same players sign their next contract.
If a house is worth $250K today, but a better house in the same neighborhood sold for $200K 2 years ago, you can try arguing with the seller that he should sell you the house for $180K because it isn't as good as the house that sold for $200K 2 years ago, but that's not reality. The market changed, and the price will be reflective of current market conditions.