the answer is simple in my opinion because they don't understand and not focus on Bitcoin, they will not understand that Bitcoin is valuable if they do not understand and know Bitcoin. there is a saying "don't know accordingly not love".
Yes, they say that bitcoin is worthless because they have lack of understanding or never been experienced of crypto investment and other thing is when they are new to bitcoin and buy it during its peak value last year and now moaning because of its price dive and with their frustration they think and say that bitcoin is not worth it to buy for.
Mostly because they only see and make decisions based on what is happening. For example, when a downtrend occurs, they will be busy reacting that bitcoin will soon die. And when the price of bitcoin soars, they will change direction by saying that bitcoin is everything. They make decisions based only on their momentary emotions. Most of the bad comments about bitcoin lately are just expressions of disappointment and not based on deep analysis. So, we shouldn't be worry.