All I can say to that is I can't predict the future, so I choose to act today. The "ones suffering in 50 years" are theoretical, the ones suffering today are real people.
Also, perhaps the people I help today will go on to ensure that we don't need to help as many people in 50 years.
As others have said, and perhaps in terms that will resonate with you, investing today provides compounding returns in the future.
And that applies to aid just as much as it does to money.
Let's take your example to a bit of an extreme, for argument's sake:
I make regular charitable donations, and plan to continue to do so for X years (X being decades), therefore helping any number of people for those X years, potentially saving many lives.
I am now standing in front of a burning building, and I see a person through the window unconscious on the floor.
I can:
(A) Put my life potentially at risk and go into the burning building to drag that one person out and save their life. If I die while trying to save that person, then that means my charitable giving stops.
Or I can (B) definitely not risk my life to save that person, therefore guaranteeing - at least for now - that I can continue to make charitable donations into the future, possibly saving a much larger number of lives.
If you take a detached/theoretical view of this situation, then (B) seems like the better choice in the long run. I will benefit more people.
But it also means I make the choice to stand there and watch that real person die when I could have done something about it.
And I wouldn't be able to do that.