It maybe the richest country on Earth, but it also has the largest wealth inequality - the overwhelming vast enormous majority of people live in poverty there, 99% are one accident away from being homeless - given the total lies of their healthcare system artificially increasing prices to impossible levels to force insurance on people.
As for those in the affected areas of LA - while some 'wealthy' they are billions of times closer to being homeless than a true billionaire.
The top 1% of America own the majority of Americas wealth, think about that for a moment, it's disgusting.
In conclusion, due to the selfish nature of their culture after decades of moulding its people for profit, those with the money to help dont care, and wont donate much of any significant sums.