Because we aren't confronting our fears,,,and aren't addressing the problem which is very, very complex. It was very hard to do, and I still do it (change words) but with practice it'll become a reflex action to NOT change the word (& perhaps struggle in the start). In a sentence, we have to remind ourselves "So what if I stammer (a bit)" (sometimes we can get stuck and opt out to change the word😬) and for me (most times) I can say the word with minimum effort...by breathing, or "feeling" the block subside.