Why is this the case? I hate how the world is like this
Good question.
It `s just the way it is. It's not entirely people's fault. We simply adapted to the environment. This is what evolution wanted :).
Survival instinct, strength, selfishness, etc. They have been profitable in the past and have allowed us to survive.
The world doesn't make anything easier.
No. Happiness isn't a limited resource.
Think about it in this small scenario: imagine giving someone a compliment. This makes both people marginally happier at no expense to either party.
If anyone needs to steal other people's happiness to be happy they're just a dick.
I think the OP meant happiness on a global scale, although I could be wrong.
On a small scale, most things are easier to do. Of course, you are right that small things can bring joy to both parties, but in my opinion this is a simplification.
Many factors influence our overall happiness, and small things can make us feel better, but only for a while.