The rich person: "What's $5 for a tip? It's nothing, so why bother."
The poor person: "$5? That's a lot, and I can afford it, so let me give it."
Basically rich people value money less, and it doesn't occur to them that giving small amounts can still help people. And obviously it's not the custom to give large amounts as a tip.
Poor people don't realize the predicament they're in and give away their money because they think someone needs it more, yet they're at a very high risk of becoming someone who cannot live independently and then needs to go find another source of charity to support them. This does not scale well. I understand why it happens but it's not something we should be raising to a level of sainthood.
The need for charity is a symptom of a dysfunctional social system and people falling through the cracks. There shouldn't be any mothers earning 19k in the first place. Giving them $5 doesn't fix anything, because you know there are a few million more of them and when you really want to change things it's really not enough and you need to attack it at the higher level.
A bone to the dog is not charity. ...
Charity is the bone shared with the dog,
when you are just as hungry as the dog.
-- Jack London