Big companies are not willing to put their capital into risky projects. If the average new product success rate was 10%, that would mean 90% of their investments would be wasted. The remaining 10% would need a phenomenal ROI for that strategy to succeed. Try selling that plan to any board of directors.
Instead, they wait for some individual or small start-up to take the risk and some sign of market interest to appear. Then they snatch them (or just the idea) up. [snip]
That's because the innovators get too attached to their creation, particularly if its a success. A better strategy for such people would be to spin off everything they created in return for a cut of the profits and them move on to the next idea.
The problem with this plan is that its difficult to make a decent living off of an invention shop. A few people have done it. Dean Kamen for example. But this requires the resources to defend your interests in your IP. Typically, an inventor receives a mere pittance for work without a demonstrated market. It might be enough to live on, but its a pretty uncertain career.
Many people, once they create something that looks like in might sell, tend to pursue the product through development and marketing since this tends to be a more stable (and lucrative) strategy. Unfortunately, this distracts the individual from putting efforts into developing subsequent innovations.