People may not act outwardly selfishly, but people may not realize that their individual action is selfish from a game theory perspective.
Everyone takes the highway because it is the fastest way to travel. However every additional car does slow it down. And this will continue until travel times on the highway either match non-highway travel times or the road literally can't handle more people. But no one thinks that driving the car to the grocery store across town to redeem a coupon is "selfish", because everyone thinks they're in traffic, not that they are traffic.
Everyone takes the highway because it is the fastest way to travel. However every additional car does slow it down. And this will continue until travel times on the highway either match non-highway travel times or the road literally can't handle more people. But no one thinks that driving the car to the grocery store across town to redeem a coupon is "selfish", because everyone thinks they're in traffic, not that they are traffic.