Why is the question mark sometimes placed outside the quotes?

Question marks and whether or not we place them on the inside or outside of a quotation depends on whether the person (or thing [a traffic sign or welcome sign for example]) quoted is asking a question. If the person (being quoted) is asking a question then the question mark is placed inside the quotation marks. Like this:

Harley asked, "Can I have dessert before dinner?"

Harley asked a question. What Harley said is within the quotation marks because she asked a question. The fact that she asked a question is indicated by the termination of the quote with a question mark. Here's a picture that might better explain the concept:

Image is my own work, edited with Google Drawings.

If however the person being quoted is not asking a question but the entire sentence is in the form of a question then the question mark is placed outside of the quotation marks (we sometimes use quotation marks for chapters of books, song titles, or newspaper/ magazine article titles). For example:

What do you think of the line "beautiful scars on critical veins"?

Let's dissect what's going on here. So the person that's speaking is asking a question (in this case someone's opinion) however the line "beautiful scars on critical veins" is the specific statement the speaker want's someone's opinion on. Again, here's a picture to help us better understand this:

Image is my own work, edited with Google Drawings.

So if the question mark is on the outside of the quotation marks the entire previous sentence is a question. If the question mark is inside of the quotation marks then only the quote is in the form of a question.

These same rules apply to exclamation points. Unfortunately other forms of punctuation such as periods , commas , and semicolons are more complex with regards to their rules.

I hope this helps!