All of the above.
I can only speak for myself, but I'm increasingly isolated because with increasing age I've acquired increasing discretion. I would love to collaborate with kindred spirits, if I could only find some.
And I don't just mean folks with similar musical tastes. I also mean musicians who actually want to make some money with their music, and have enough practical sense to realize that success doesn't happen by magic. And I especially mean musicians who don't insist on bringing their politics (or other religious beliefs) into the equation. If we're going to make music, let that be the focus. I don't give a rat's behind what your political prejudices are, and frankly, I don't even want to hear about it.
Your point about intellectual property is especially relevant in my case. There's no shortage of people who find my musical and production skills, experience, and ideas valuable. But when it comes time to apportion the compensation, suddenly I'm expected to divvy it up "fairly and equitably"...meaning I write most of the songs, put in long production hours in the studio, and in many cases end up adding much of the instrumentation, but then I'm "selfish" because I disagree that Equal Shares For Allâ„¢ is a rip-off. It's not "democratic", you see.
The answer is a contractâ€”a voluntary, mutual agreement that lays out IN WRITING the way in which everyone's compensation is indexed to their actual contributions. Well, you wouldn't believe the resistance to that idea. If you try it after you've already invested the work, you're accused of "changing the deal", even if you had a verbal (oral) agreement in principle. That's why you have to get it in writing.
Trouble is, when I've proposed it up front, no one wants to do it because of some bizarre, misguided, utterly impractical world-view in which a contract is anathema...perceived as some kind of capitalist exploitation, so some such nonsense. "I hate that sh_t! I just wanna make music..."
And the younger and less experienced people are, the more likely they are to be put off by a contract...and of course, it's simply unthinkable (to them) that there should be any consideration whatsoever for providing the value of writing the contract. Why should there be? They don't acknowledge its value in the first place.
So, it's just easier to do everything myself...but there's the isolation. And the inspiration and motivation of collaborating is the first casualty.
I've actually considered creating (yet another) online clearing house to foster collaborations. I think I know how to do it right, having been instructed by the lessons of several past collaborative failures.
But frankly, I don't want to entrepreneur such an operation...not by myself, anyway. I would consider contributing my skills and valuable experience acquired through the school of hard knocks to a joint venture or partnership, but I don't want the hassle of being responsible for the whole enchilada.
The truth is...well, "I just wanna make music..."