Why am I (23M) always just a friend?

I have gone out for coffee with about five different girls over the last three years and each of them has gone well; Each one of them indicated that they'd love to hang out again, and each of them has texted me a few days later saying that they'd prefer to just hang out as friends.

I've been asked repetitively by people "how I'm still single", even by the girls I go out with. I'm considered by my friends and coworkers as funny, friendly, and kind and so I'm worried that I offer enough as a friend as to not warrant an "upgrade". I am confident about myself, but each time this happens I can't help but see that the only common denominator is me.

I know that this isn't enough information to figure out what's going on, but at this point I'm so lost. Does anyone know why this may be? Feel free to ask questions and I'll answer them to the best of my ability.

Thanks in advance.