I want to qualify this with I am not a professional, I just do commissions here and there for friends and family. But here's my best guess:

Until you know what the project is, it's hard to set a price. Sure, you could list an hourly rate somewhere, but a lot of clients will then come into the conversation thinking "well that'll only take you 2 hours so I'm not paying more than x" when you know it'll actually take 6. Or perhaps the project in question WILL only take 2 hours, but requires a lot of specialized materials that you need to purchase. There's a lot of things that can go into determining the price of your work, and until a conversation is had about what exactly the work entails you can't know what's a reasonable fee.