At the end of the day, it's the same reason most people go broke: they know nothing about money. Unfortunately, Money is the most important thing in the world because you can't get anything without it. And yet, it's not even a primary subject taught in k-12. Where are people supposed to go for a decent financial education? Instead k-12 teaches kids about useless butterflies and countless other things that are of less…
Oh, I think they probably know the basics.
They know how percentages work well enough to know that they could take all that money and put it strictly into safe investments, and get a considerable, steady cash flow from that, on which a more than comfortable life is possible.
Any rank and file financial advisor at a local bank branch could set them straight on the basics, if they want.
Come on, just half a percent earned on $100M is $500,000; total no-brainer.
That's boring though; they want to substantially multiply their wealth. They want to take their dough and blow it sky high. E.g. guy who has a $100M million put away thinks he can turn it into a $1B empire.
It's like another sport; scoring another goal.
In some ways, taking the risk makes sense (just not to the point that you go broke). Say you have that example $100M, and you could earn 0.5% on it for the rest of your life. Well, whoopee; why would you? You could simply draw 500,000 a year from that for 200 years, even if it earns nothing. (Let's ignore inflation and all that for simplicity.)
So you can see why the lower-yield safe things are not attractive if you have a lot of money.
I think these guys just fall victim to a "go big or go home" thinking. Plus they probably think that they have so much money, they don't have to keep tabs on exactly how much.
Also, while they have those high paying contracts, they think that their income can always save them.