Short answer-no they are not foolish, some are greedy, some naive, some reckless. but over the last 3 years bitcoin has shown a steady overall upward trend in its price. People have jumped in to crypto without fully understanding it, hoping to make a quick profit-thus overheating the market and prices.
To buy bitcoin or any crypto with borrowed money is just plain silly, the only thing you should borrow money to speculate in is real estate and even that has risks.
All true, one correction only if I may: the slow and steady overall upward trend is present over last 9 years since the famous "Chancellor on brink of second bailout for banks". The last 3 years is the time of still wider adoption, being on brink of "popular adoption".