In addition to other answers, consider that in respect to something which can be considered “more” or “less,” the term “extremely” can refer both extremes. When you want “highly” it often applies only to the positive extreme.

What makes people “highly sensitive?” ” The same might be described as “extremely sensitive. ” Is it illegal to describe something as “highly insensitive” while “extremely insensitive” doesn’t carry the same awkwardness?

How can “highly” and “extremely” be applied to riches as a second example? What does be considered a highly poor person, and therefore a “highly rich” person? “Extremely,” by contrast, is applicable to both.