Only unpopular or unscalable software projects are done in a few months. What you're describing is already the norm for custom internal software or other categories of software that aren't necessarily designed to scale beyond a specific purpose. This kind of work is often outsourced to contractors who are hired for months. But the reason that these projects have a definite time horizon is because they are seen as the cost of doing business, not a core revenue generator.
Successful, scalable, revenue-generating software projects that compete in a big market, on the other hand, are never finished. Microsoft still has top engineers working on Excel and Windows. Google almost certainly has some of their best people incrementally improving Search and Maps and Gmail. Many of Amazon's top engineers are working on improving AWS. Apple's certainly not transferring their best engineers off iOS.
The economics of software competition is such that making even a very small improvement to a successful software project that scales to millions of users can be worth billions of dollars, while creating entirely from scratch something that is half as good as another extremely valuable software project can be worth close to nothing. This means an extremely large number of top engineers are going to be working on incremental improvements to successful products purely as a matter of economic necessity.
Also, the idea that top-level engineers should constantly move to new projects is superficially alluring because new projects are supposed to be fun and challenging and the best people should be rewarded with fun and challenging tasks, but in the real world, engineering is mostly about constraints. From a product standpoint, coming up with a new product is challenging, but from a software engineering standpoint, greenfield development is not very challenging because there are so few constraints that make engineering difficult.
Edit: I'd also add that in companies that specialize in the type of projects I described in the first paragraph, what you're advocating is pretty much what happens - the best developers in non-product companies often become architects and managers who get to make technology choices, talk to clients, produce specs and/or working prototypes, while their less-talented, lower-ranking peers are left to deal with the details and implementation issues. When quality isn't important, this is efficient.
Edit2: I think part of the reason why people get the impression you do is that most people's greenfield projects are disproportionately fun hobbyist projects where they get to be the sole guy in charge, whereas most people's non-greenfield projects are disproportionately corporate projects where they are subordinate to the wishes of many others, including other developers, managers, product people, customers, designers, etc.