Because they have more money that they can spend, while you an other human being is almost starving, and maybe can't access to education or healthcare.
It's not only a matter of billionnaires. We can wonder if it is fair that we (most of us are middle-class of developed countries) live a life of opulence when others are struggling. It strikes me whenever I travel in less developed countries.
If everyone in the world could afford 1 yacht, but 1% each had 1,000 yachts, that is mass inequality. But would that bother you?
Land, human labour and political influence are three of the most rivalrous goods, goods where the price goes up as fast (or faster) than the supply of money.
In a world with little poverty but vast inequality, everybody could afford TV's and video games; food would be expensive but affordable. Labour intensive services like education and health care would be unaffordable. Owning or renting a nice place would be unaffordable.
(I am not an economist)
The problem isn't unequal yachts, its when that same mindset is applied everywhere else in society, to political power, judicial protection, civil rights. "Why are you bothered by police gunning down unarmed black teen boys, you're not an unarmed black teen boy and the cops haven't done anything bad to you so far, you're getting enough justice its just that some small fraction get much less justice and thats OK because they're not you"