I think it's because they're in a rush and don't want to wait for the light. They're not thinking of anything but where they are going at that moment, so they have no consideration of the consequences that their action may cause.

In no particular order: 1) They're in a hurry. 2) They believe the car and truck drivers will allow them to finish crossing, since pedestrians have the right of way. 3) They're preoccupied, so they aren't paying attention. 4) They're tired and don't care. 5) They're tired and halfway hope someone in a car will bump into them so they can sue the driver, or at least get a ride home or to a bus stop. 6) They're adrenalin junkies and like living dangerously. 7) They're time travelers and happened to plop down into the crosswalk when the light changed. 8) They're from a galaxy far, far, away and on their planet, 21st century Earth cars and trucks were dismissed as fantasy fiction, so they were caught off guard when they plopped down in the crosswalk. 9) Someone dared them to play chicken with traffic. 10) They're stupid. 11) They aren't stupid, but have never used a crosswalk before and don't know the rules. 12) They're color-blind and also crossing at an old intersection that does not use the "person walking" symbol. 13) It's mid-day and the sun's rays wash out the traffic lights. 14) A combination of 12 and 13. 15) They're hallucinating. 16) They're insane and believe that their bodies are made of a substance that will turn bumpers and metal into Silly-Putty and the vehicles will therefore bounce off, while they, the walkers, enjoy an unimpeded crosswalking experience. 17) They're inconsiderate. 18) They enjoy frustrating drivers, especially if they don't have cars themselves. 19) The walker is in the middle of a panic attack and the only thing on his mind is getting someplace safe, even if it kills him to get there. Sort of related to #1. And finally, (thanks to john pennington!)... 20) They're yakking on their cell phones and aren't thinking about traffic, crosswalks, or vehicles and their drivers.

Sometimes the obeying of laws is unknown to people who come from other countries, like Mexico, where the rule of who does what is settled by a fist hitting a face or a knife cutting someone or some other means of force.

Their actions say, "the seconds of time I am saving are worth more than another person's life." They may not be conscious of that reasoning, but their actions speak it loudly.