Because that's what tournament play is. It's about winning the tourney, and the only tourney which pits all the nations against one another is the WC. That's why it's really the only ODI tourney which matters. It doesn't mean you can't enjoy ODIs at other times, but I reckon if you asked the players, the vast majority would take winning nothing but the WC over winning every other ODI between WCs but crashing and burning on the big stage.
And there is no point playing countless LO series between WCs, other than to raise money, which tbf, is a not unreasonable motive.
I don't feel it is that black and white. The reason players even become good is due to experience, and that too experience in a competitive environment. The fact that a player averages over 40 in ODIs over 200 some ODIs says something significant about that player, and so do the ODI team records about the respective teams. The world cup is the pinnacle, but you people are being totally unfair in reducing the hundreds of other ODIs played as rather irrelevant and meaningless. The fact that Pakistan has a very good ODI record against India overall is not meaningless, some even say the six Miandad hit in sharjah went onto have a significant impact on both Pakistan and Indian cricket respectively thereafter. In the end we have to realize that the world cup itself is an incredibly small sample, it tells you about a teams ODI pedigree but it also leaves out much.
Someone posed a question whether most Pakistanis would switch records with India due to their world cup failures, perhaps they may, and it might just be an emotional response, but would they be willing to accept something like 0-124 outside of the world cup for 6-0 inside? I doubt it, that would mean tremendously under-appreciating great cricket and cricketing moments that have spanned decades, it would be unfortunate and many great players would be forgotten. Tendulkar desperately wanted to win a world cup, and he was very close to retiring without winning one, despite this he still would have went down as being one of if not the greatest ODI cricketers of all times.
It would be different if we were talking about something of a more appropriate cricketing measurement such as a test championship of sorts, in which the best teams would almost always win, but these are ODI tournaments, and that too once every 4 years, luck does play a greater role, I don't think they mean and determine everything, they are a great spectacle and make for great competition. The world cup is the greatest cricket tournament of all, but it doesn't render everything else useless.
As far as money is concerned, it is all a cash grab, even the world cup itself, the ICC has demonstrated that it couldn't care less about the game itself by bringing the hammer down on associates.