1. Your idea probably isn't as great as you think
I've seen a lot of different people who had "game-changing" ideas, at least so they though. A few of them actually managed to convince me and my partners that their idea is so amazing that it'll kick everyone's ass. Even though proper market research wasn't done, charisma is sometimes hard to resist, and if you are working with someone you've known for a long time, you are prepared to accept crazy terms, such as a delay of payment until this idea will start to generate revenue. After these specific ideas were put into the real world, it turned out there is a huge discrepancy between ideals and reality, and we ended up with unpaid invoices and ignored phone calls.
Ideas are something, execution is everything else. There is a long way inbetween, a way paved with upgrades, downgrades, changes, pivots, time and hard work. Millions have ideas, only a few can make them work. I've seen Seedcamp companies change their core concepts and business models completely, and these startups are already the best, selected from hundreds, if not thousands. When you start working on something and proceed ahead, the initial idea will often evolve beyond recognition. Not to mention there is a very strong possibility that someone else was already thinking about the same thing, except better, years earlier. Your idea isn't amazing, but it may be good enough to achieve something with proper execution. That's why you need feedback and partners.
2. I have plenty of ideas of my own
You know what's better than your idea? My idea!
Mostly because I kick ass, but also because people get emotionally attached to the thoughts they generate by themselves. I have so many ideas I don't know what to do with them. They are probably not really great (see reason 1), but they are mine, and I try hard to make a few of them come alive every now and then, when I have the time. During the day, I work on real-life projects, during the night, I play around.
When I decide on what to work on next, I usually look for the best ratio between actuality, complexity, required energy and potential. This means I've already made my own priority list of the services I will be rolling out in the future, and I must say, it would really be hard to put one of yours inside this packed list. I'm sure most developers think in a similar fashion, lacking resources to make everything they imagine a reality. Face it, there are hundred times as many people who have unrealized ideas, than people who don't know what to work on. Do the math.
3. Your idea probably requires specific passion and know-how
The idea lives strongest in the person who thought of it. It is a result of that person's experience from many fields, so it's hard to replicate in its full form without that experience. The ideas I've stumbled upon usually solve very specific and niche problems you can't solve without digging yourself into that field. Which most of us don't have time or the resources to do. The core of the idea represents the person who thought of it, it may be taken to another level by a different person, but in most cases, it requires the original author's knowledge, involvement and passion to work as it's supposed to.
I can't execute an idea which will revolutionize kindergarten children education, I don't know shit about the problem, I don't have any connections in the industry, and I'm simply not that passionate about that field.
4. Your idea requires your involvement as a product manager
In the past few years, we've implemented quite a few prototypes and services together with Neolab, and even though a few of them got some praise, they didn't make it to the mainstream. Take this blog for example, I developed it in early 2009, aggregating posts from different social services, presenting them (also) in a magazine form. What did I do with it? Nothing. Years later, a service called RebelMouse did something similar and raised millions in funding. Same goes for Twenity, a spin-off from Twitfluence, gamifying social authority measuring. Or Ljubljana Realtime, a social event discovery tool. All out there, but that's it.
I simply don't have enough energy to push a service beyond a point, or don't want to. Perhaps this fact will change someday, but at this point, you will need to be the product manager of your idea, and I can be its architect. Since I have a real company to run besides all of this, I can't afford to be one.
(Btw, if you think you could do anything with the above mentioned things, don't hesitate to give me a shout).
5. Karma and stuff
I believe in karma. Don't do evil and all of that. I would really feel uncomfortable if I would take someone else's baby and make it my own. So I won't, because this simply wouldn't be a fair thing to do. I value proper sleep above success.
But what if
Of course, there are always exceptions, and I can imagine I could encounter something that would go beyond all of my points above. Perhaps there is one idea that I've heard about years ago that would suit this description. If I ever decide to proceed with this project, I will let that person know what I'm doing and invite him to join the project. Even if I'm thinking about a thing that only faintly resembles the original concept, I can't deny it's that person's idea. And since this guy was able to think of such a marvelous thing so much time ago, he would surely make a great addition to the team (also see reason 3).
That's it. I won't steal your idea, so feel free to talk about your revolutionary innovation with me anytime. All I will do is to try to tear it apart and put it back together, and after we're done with that, if I get the chance, I will try to bring it to life.