Because with $1,000, that might be the money you need for rent - $65 could be the prescription you need til the end of the month. Despite being 6.5% of the amount, it could be the difference between you actually eating that week.
But a person with $1 billion has their every need and desire met. They don’t have to worry about rent or food or medicine. They could fund the research for the cure if they caught a never-before-seen disease. $65 million may be 6.5% of that, but it will make no difference in their ability to support themselves.
tl;dr It’s actually less morally defensible for the billionaire to keep the money
