“Most people” is a strong way to put it. If it didn’t work for most people, why would it be an entire industry and field of study? As a person who’s taken 3 different kinds of speech therapy at different ages, in my experience it comes down to the person. Some people have a more severe stutter than others which unfortunately, just might not be possible to reduce. Others might be too young to really internalize what’s being taught by the therapist. My experience having speech therapy as an adult vs as a kid was vastly different because I took it more seriously and was self aware enough to see what was working and what wasn’t. Lastly, there’s a lot of different approaches so it’s up to the individual to shop around for types of therapy and professionals.