Do you ever get the response?

There are a couple of reasons.

They don't want to scare you away. They don't want to hurt your attempt. Answering straight "I've got a boyfriend" sounds like she's not being open-minded , or the other person could misunderstand it as "I've got nothing to do with you, not even trying".

Also, girls want attention, as much as possible. She can only have one boyfriend but it can't harm to have others worshipping her as well.

In addition, it would be rude or even arrogant to assume the guy wants some relationship only because he is asking this question. Maybe it's just some light talk and easy get-to-know.

Or they tell the truth.

Jam it back in, in the dark.