I don't think that's an answer but passing the buck. Why are they separate in the first place? Why aren't record labels, as part of standard modus operandi, been buying copyrights from bands? They are best placed to do this sort of thing, much better placed than random musicians who happen to get rich and then dabble in investing - record labels have (or had) the contacts up front, they had the money, the diverse portfolios to spread risk...

I think some of this stems from history. There was a whole music publishing industry far before 'record labels' existed (before recording existed, really).