Of course, that's also not often a choice I (have to) make. I've a set amount of donations to a few regular charities, whereas a situation as above would "count" towards my regular daily expenses rather than charitable givings.
It's probably also related to always being able to do more. Yes, maybe giving €x to one of the Givewell organisations is more effective than giving it to Make-A-Wish, but often it's not a game of either-or. I'm sure a lot of people donate to Make-A-Wish because they encounter it and sympathise with it, rather than spending effort looking for the most efficient way to spend <arbitrary amount designated for charitable donations>, so it's not even a question of effective charity vs. MAW, but MAW vs. sitting there in your savings account.
People only investigate a finite number of things, adding just 1 more to the early stage to the millions already tried is not likely to be useful, but we simply don't know.
PS: After that point much larger sums are involved, but even then only a finite number of tests get done and they are very dependent on funding.