First: yes, there are ethical reasons to accumulate so many resources: because some problems are incredibly hard to solve and need a lot of resources to even start to solve.
Second, it is not on the people who have earned something to prove why they should be "permitted" to keep it; it's on the people who believe it should be taken away from them to justify that. Saying "it's a shame that there are billionaires" is a complaint that some people are wildly successful and shouldn't be, or that they shouldn't be permitted to keep what they earn. There is justifiable resentment and offense to a conversation that starts with a thinly veiled demand for justification, rather than a presentation of justification. The burden of proof is approached the wrong way, and nobody seems to discuss that.
(I'm not interested in quibbles over the word "earn" here.)
It's not a shame that there exist billionaires. It's a shame that there exist people with nothing. The former doesn't need fixing; the latter absolutely does.