RTEMS.org has been around as an open source project since the early 90s and I agree with you. It is the responsibility of the core developers to be architects, build the basic structure, and add enough value to that where others can use it but still have clear areas to contribute.
But that isn't all that is required to get contributions. You have to have low barrier to entry for both users and new developers, be nice and help them, be responsive to reviewing and merging, and have decent documentation. You have to have a focus on continuously improving that experience so it gets easier and easier. Try to avoid having different people asking the same question over and over. You aren't getting paid on a per question basis.
If you are lucky enough to have a project that requires some customization or assistance in deploying, you even have a path for some consulting revenue. Since RTEMS is a real-time operating system for embedded systems, there are always new boards, custom boards, new CPUs, new peripherals, etc. that need to be addressed for users.
Sometimes users pay for major feature additions but the core developers still need to have a roadmap and often do the groundwork for that feature in anticipation. And it is important to make it known that some features are highly desired. Having open projects and desired features available in an easy to browse form can also attract people who just want to work on some open source project or students who realize your wish list items relates to their projects or research.
It is also critical to automate as much as possible especially testing. It can become a huge time sink for things that have to be done manually and that detracts from improving the project, vetting contributions, and helps burnout people.