Because we don't need money, currently. Haiku is not a company. It is a project people are contributing to for fun and by passion. It could go so long without any management because of this. Over the years we have assembled a team of people with a common vision about what the OS should be. You cannot achieve anything similar by hiring random devs. So, Haiku as a company would simply not work.
This is why our answer is always the same when someone wants to do something like that: fork our sourcecode and create a company. I think most Haiku devs would be relieved to know someone is working on the OS and would use their free time to work on apps instead. It is not an agressive rejection of people willing to help, it's just that for us, that wouldn't be the Haiku project anymore.
On getting money, the way GSoC and GCI work is actually quite good (both the GSoC students and the project get paid for their work). The most important thing we get from there, however, is new contributors (remember that I would likely not be here without GSoC).
As for collaborating with other projects, we do it on the technical side when possible, and we should do more of that. But on funding? Would people donate to a random set of OS more than they donate to a specific one? How would we share the funds? I fell it would create more problems than it solves, unless there is a real common target (let's say, writing a driver for some device that would work on multiple systems?). And then it's up to the people wanting to achieve that to fund their efforts, not to the OS projects.