Why is it that the rich can just give away their money since they've excess of it?

It's because any money given to you without you working for it will eventually destroy you. So those rich people you call stingy aren't stingy, they're just protecting you from the you that's capable of destroying you.

As such, Take your finance seriously 

Learn about how to earn, invest and multiply money daily 

Learn skills, build businesses, products and services. Build systems, processes and structures that keep cash flowing into your pocket.

I don't really agree with this as the reason for the rich being selfish.

First of all, most of the people you think are rich are not, they don't have excess, but have enough and know how to manage it, if they try to be generous, they become poor fast.

For those that are actually rich(have excess than they can ever need) are just greedy. They are not protecting you from anything.  Its just the normal human instinct of always wanting more that has kicked in, and their ego now sits on the wealth.

Only a few percentage will withhold money from you while they have excess for the sake of making you better, and they are usually family and they are always cautious not to make it dangerous but without just enough to Kickstart the fire in you to fight for your own.