Maybe his house burned down and he had failed to keep offsite backups. Maybe he never made backups in the first place and his hard drive failed. People make all kinds of mistakes, sometimes very costly mistakes.
For example, pizza-Bitcoin guy. The stories always talk about how he traded 10,000 Bitcoins for two pizzas. That's only a small part of the story, what actually happened is drastically worse. He sold several times that many Bitcoins in other low-value transactions at the same time (some straight up for cash) and did the pizza trade more than once.
Pizza guy likely traded a minimum of 50,000 Bitcoins for less than a few hundred dollars of value. That's $1.6 billion presently at 50k.