Read your own definition. It says usually. Otherwise it just refers to a gain or recurrent benefit in general. And what is a gain?
resources or advantage acquired or increased
Wealth/money is a resource and/or an advantage. And yes, when they get the inheritance, they acquire it.
If your father hands you $10,000 in cash, would you report it to the government as income? I didn't think so.
What kind of American are you that thinks the government is more deserving of ones wealth after he or she dies than the family? You ask what the kids did to earn that money, and I ask you what government did to earn that money?
What exchanges happen within the family are family matters--not government matters. Perhaps you want government sitting at your dinner table, and when you pass the biscuits to your daughter, government is there to take their few biscuits first before they pass the plate to your daughter. It's ridiculous.
People work hard to try and make their children's life better than theirs. They didn't go to work every day, take risks, only to hand their hard work over to government.
And learn how to read. The definition is usually measured in money
that derives from capital or labor. I even highlighted the important part of the definition.