Well, I teach English in a university. It's my job.
Can you think of any reason why I would want to spend my free time teaching someone else?
If I wanted to learn another language, maybe I would find someone who would exchange classes with me. I teach them, they teach me. This is quite common, and there is a section of italki for this.
I guess there are also people who want to show off, or want to meet pretty girls online (or handsome boys), or want to practise teaching. Teaching something is a good way of improving your own skill.
But everyone wants something! What can you offer in exchange for help, advice and the opportunity to practise? You are getting something of value. You should offer some trade.
Personally, I teach for money. BUT why do I answer questions here? Am I hoping to find more paying students? No, I answer questions here because I like doing it. All I get is satisfaction, and most people do remember to say thank you.
So, you don't always have to pay, but you should be ready to offer something in exchange.
Best of luck with it.