Not entirely true, most of those billionaires put very little of their own money on the line even in diversified projects. A lot of billionaires are deeply invested in concentrated assets and use them as collateral to obtain bank loans and other financing to engage in riskier projects. How much of Trump's money was/is invested in his projects? Very little. Most sits safely in the bank collateralizing debt he used to finance his "projects". Very few billionaires go all in unless it is their own baby. One such billionaire is Michael Bloomberg.
They invest in real estate, develop projects worldwide in mining, excavation, drilling, alternative energy, space travelling, restaurants, venture capital, wine-making, horse-breeding, politicians, basically spread their money around so as little as possible is put in one basket. They don't just have money in the banks; they own the banks by sitting on their board.