Most organizations rely on outsiders who contribute their time, money, expertise and ideas, and they gladly welcome such contributions. By getting involved with an established organization, individuals can often take advantage of its existing resources, increasing the likelihood that whatever is contributed will produce an ROI.

Obviously there's no absolute guarantee of this, and I wouldn't suggest that there's no room for new organizations, but new non-profits are a lot like new businesses: most do not "succeed", and a lot of the ones that don't fail outright plod along and never achieve what was hoped for. It doesn't take more than a quick read of a few random Form 990 filings to see this.

So to put it simply: you do not need to start your own thing to help solve big problems. People who have the itch to save the world would be wise to consider that they are far more likely to make a bigger impact by channeling their time, money, passion and ideas to organizations that have been saving the world for years.