(1) Some problems are probabilistic either in theory or practice. For instance there could be a sentiment analysis problem where the state of the art was 67% accuracy 5 years ago and with an LLM it is easy to get 85% accuracy. 100% accuracy is going to be impossible anyway because sometimes you really can't figure how somebody feels.
(2) It's a big topic that could be addressed in different ways but I'll boil it down to "people are sloppy" and that many people become uncomfortable with complex problems that have high stakes answers and will trade correctness for good vibes.
(3) LLMs are good at seducing people. To take an example, I know that I was born the same day as a famous baseball player who was also born exactly a year before an even more famous cricket player. I tried to get Microsoft's Copilot to recognize this situation but it struggled, thinking they were born on the same day or a day apart rather than a whole year. Once I laid it out explicitly and my own personal connection it had effusive praise and said I must be really happy to be connected to some sports legends like that, which I am. That kind of praise works on people.
(4) A lot of people think that fixing LLMs is going to be easy. For instance I'll point out that Copilot is completely unable to put items in orders that aren't excessively easy (like US states in reverse alphabetical order) and others will point out that Copilot could just write a Python program that does the sorting.
That's right and it is part of the answer, but it just puts off the problem. What's really irksome about Copilot's inability to sort is that it doesn't know that it can't sort, if you ask it what the probability is that it will sort a list in the right order it will tell you that it is very high. It's not so easy to know what is possible in terms of algorithms either, see
https://en.wikipedia.org/wiki/Collatz_conjecture
as evidence that it's (practically) impossible to completely understand very simple programs. See the book
https://en.wikipedia.org/wiki/G%C3%B6del,_Escher,_Bach
for interesting meditations on what a chatbot can and can't do. My take is that LLMs as we know them will reach an asymptote and not improve explosively with more investment, but who knows?