Simple economics. Supply and demand. It's how the Colts value him as a franchise quarterback versus available alternatives at the position.
Wow contracts are not unique to any one particular sport or isolated to the sports industry as a whole. If professional athletes' contracts were truly based on talent, even the most perceived talented ones would be garnering agreements valued less than eight figures per year in my opinion.
It's always about how much those with the means are willing to pay. Is anyone today worth $50 million dollar contracts, $100 million contracts, $200 million dollar contracts, etc? Not really.
Besides, being wowed is relative to what people think are huge sums at any given time. If minds could be wiped, people placed in a time machine and sent back 50 years would be wowed no less by Joe Namath getting a $400,000+ contract. Same goes for the future but in every era there is one constant: economic supply and demand.