Originally Posted by haolaowai_org

Because then fares would go up by $X, which means passengers will now to to another airline that has slimmer seats but prices lower by $X.

http://flightdiary.net/chineseman/2016

I fly EXCLUSIVELY cheapest Economy, no upgrades to seats. I get what I pay form and I know what I'm paying for

Did you know that with great shoes, you will reduce your chances of injury by X%? Why doesn't everyone buy the healthiest shoes for $2000/pair? Because some people trade that risk of injury (albeit small) for lower cost. The same applies to seating. If people VALUED not getting DVT above all else, they would either not fly or buy a private jet to fly (extreme example).
The fact that they fly, and fly economy, means that the risk of DVT from a small seat isn't worth that extra $ for more room.