This is not always true btw. Maybe it is most of the time but really should be case by case. I went through a situation where none of this traditional advice was correct. Sometimes in corp jobs with budgets outside of managers control etc an outside offer opens doors for counter offers that otherwise don’t exist. And if you like your team etc and know you aren’t being replaced, I don’t see the harm in staying in specific situations. But if you don’t know or are unsure then probably in general good to not take the counter