If you're suggesting that this is done out of selfishness or intellectual fervor then I don't think that's true. I wouldn't say they "prefer" it but it's certainly easier and more economical for a skilled person to contribute to those tools because the developers and the users are the same group of people.

Once you decide you need to do user testing and hardware testing, the cost shoots up. You need to start procuring hardware and finding a place for it to live so automated tests can run on it all the time. That costs money. You need to bring in a lot more testers which requires project management and/or volunteer coordinators. Usually that costs money too. Everything you do needs to go through more stages of design review which also requires extra coordination and project management. These are real things you can have but someone's got to foot the bill.