Because if it was legally income, it is a crime not to report it. If the IRS discovers it later, they can charge you the taxes, plus interest, plus penalties, and in some cases prosecute you criminally (see Al Capone). Whether it needs to be legally reported as income or not is not determined by whether someone else already filed paperwork for it or not. I don't know if this would need to be reported as income or not, under what circumstances; I'd talk to a lawyer or CPA. whether someone filed a 1099 or not is not determinative.
I'd talk to a lawyer or accountant when dealing with a mystery $250K instead of making "starting assumptions". It's certainly not determined by whether a 1099 was filed.
If you run a business, do all of your customers send you a 1099 for the money they pay you? No, that would be wild. Your revenue is still income (corporate or individual income). 1099s are only applicable for certain situations.