Friend I don't talk to sends me a bday gift every year

I was really close to a friend of mine when we were teenagers, but have since fell about of touch. (We didn't have a falling out or anything — we just don't hang out or call each other anymore because life got in the way.) But, every single year, she sends me a really nice birthday gift, even if we hadn't spoken one word to one another in the previous year.

I'm trying to figure out what's behind the gesture: Is she looking for one in return? Is she hoping to breathe life into the friendship again? Or does she simply enjoy buying gifts?

I feel guilty because I simply text a thank you, but never send her anything in return because... well, we don't talk. Thoughts?