I Don't Get It, I Don't Just Don't Understand

i occasionally receive a private message from a new member, usually male, but a couple have been women, or said they were.

Some simply say "I want to get to know you"
To these, I invariably reply, "Why?"

I don't have a photo of a woman as my profile picture. I don't state my age. So, I ask why they've decided to know me. I'm curious. I really want to know why they chose me. Each one has become offended by my bad manners.

Why is it rude to ask someone what they find interesting about me?