Because it would take an order or magnitude (if not more) of time to carefully demolish the bridge by taking care not to damage the girders. As it is, they just let the Hoe Rams go wild and pulverize the bridge. The rubble does get sorted and the spaghetti of rebar and the girders do get sent to recycling.
To preserve the girders, the concrete deck needs to be removed by saw cutting (careful not to nick the girder). Remember the girder has shear studs to connect the concrete deck to the girder, so the portion above the girder has to be chipped out by hand. Then removal of the girders has to be done by crane (which has costs and time to set up). Not in this case, but often the girders are too long and need temporary support during the de-erection process. Finally, few Owners would want to build a new bridge, and specify used girders. Even though not visible, I am sure there is rust at the girder ends (which could be chopped off and made shorter). There is also likely some rust at the interface between the deck and girder (de-icing salts and chlorides have likely seeped through the concrete deck), and along the bottom flange (where bird droppings trap precipitation and condensation and promote corrosion).
So if you were building a new bridge of similar size, the bridge would cost about $5,000 per m2 (of deck area). The structural steel is maybe $500 /m2. Would you specify used girders to save maybe $200**/m2 (there still a lot of effort required to move the girders, store them, reconfigure them, sandblast and coat, etc.)?
** - if we add the cost of taking great care in demolishing the bridge - it would cost more to re-use the girder.
