Because famous people did not create poverty and all famous people may not care so much about these issues. Further, a few people cannot be forced to correct a worldwide burden that in many instances is chosen (often without planning or unwittingly, but chosen nonetheless) by those who suffer from it. As for animals the causes of these issues usually lie within the individuals who are causing animals harm or causing them to be homeless. Often those who abuse animals have many problems, of which financial or even mental issues tend to be common. However, it must be noted that all animals cannot be matched with a home and some are downright dangerous.
In the case of people who are in poverty there are many causes, none of these causes are traceable to famous people. Poverty has existed since the beginning of human existence. If one doesn't know the absolute myriad of causes of poverty, they cannot even imagine any useful approaches, let alone take useful corrective action. Poverty can be caused by tragedies, catastrophes, crimes, accidents, lawsuits, injuries, illnesses, unplanned pregnancies, and more.
Further, poverty is often a result of a person's choices and in other cases, a lack of choices. Still in other cases, there may be a devastating combination of these two problems. For example, many poor people the world over tend to create children they cannot and/or will never be able to afford and they tend to have less education, skills or an in-demand trade upon which to rely. Most often, creating children is a choice and those who don't plan often fall into the group of people who are in poverty.
Those born into poverty are too often doomed to repeat it because their families tend to prefer having similar minded people in their lives rather than working towards a useful and needed change. In many cases their offspring often do not have access to opportunities, information, mentors, connections and skills that those from more abundance will have access to.
No one should give literally all they have to help another or give until it hurts them, that would put them into poverty. Famous people may be famous but that doesn't mean that it is their responsibility to give until it hurts them and/or their families.