“You’re such a catch. Why are you single?”
Same reason anyone is.
The people interested in me, I’m not interested in, in that way. The people I’ve been attracted to or even mildly interested in haven’t been attracted to or interested in me back. Or we didn’t want the same things/weren’t on the same page.
People who want to be in a relationship often don’t just choose not to be. So the question itself is kind of silly.
Instead, I suggest considering asking people what they’re looking for. Or just wishing them the best. Or leave it at a compliment.
“You’re such a catch.”