A few things to ponder. Folks in open source are generally builders. They like the craft of making things. They're also people with ideas and they want to explore those ideas. Open source is the perfect outlet for that.
Second, there's no forcing people to contribute to a project they don't believe in. If there was only one DE, it would be the same people working on it.
Finally, the energy and exchange of ideas between open source projects elevate every project. Without that variety, Linux would be stale, boring, and lacking ideas. It would be no different than Windows.