Because all they think about is making a big profit in a short time and only seeing the price go up and after making a purchase they expect the price to continue to go up and it turns out the opposite.
as I know the majority of beginners make mistakes like doing something without being based on adequate knowledge and analysis with their goals, and end up losing in a short time.
People seem so confident to buy asset when the price is high because they believe the bullish period will continue and they ignore so many risks that may occur in trading. This belief is possible only because the bull market this time has involved so many institutional investor that it has led many trader to believe that price are continuing to rise. But what is happening now is the opposite and the price is gradually going down and there is no certainty that it will recover any time soon.
However, there are still many people who believe that bitcoin still has the potential to grow bigger than what has been achieved now and that it will affect other crypto asset at large and make the market capitalization increase.