I guess you do not participate in casino contest a lot or maybe you are type of gambler who have decent bankroll so you can beat all competitors in a contest easily and take the prize yourself, that's why you can say that gambler do not care about second or third place.
Imagine if you are an owner of casino and you provide contest with 1 prize only and since the contest started, there is 1 player who is dominating it with a big gap with the other players.
It is more risky to join contests that have only one prize for a first competitor in final rank. Such contests are less attractive for participants and honestly I have never seen any such only one first position prize in any gambling site or any contest hosted in the forum or in real life like any award (Best scorer, Ballon d'Or and similar ones).
It can be awards for first three positions and even more than that for lower positions like top 10. It will give more chances for participants to win something from small to big awards and such contests are more attractive.