This is a tale of value transferrence: how creativity is transmuted to the base common demoninator that everything seems to get translated to: capital. Why will the copyright to most Beatles songs end up scattered to the winds, in the hands of anonymous - or big-capitalist - shareholders?
If you will, you could try blaming poor tax advice, Yoko Ono, or the extravagant lifestyle of Michael Jackson - plus a large host of minor players. But quintessentially, there were a lot of poorly-handled relationships, only some of which are detailed here.
In a publishing contract, a songwriter assigns copyright to a music publisher, a company that does the quotidian administrative work of collecting royalties for the song being recorded, played, or used in any context; the publishing company typically has control over licensing the song for use in other media. In return, the songwriter gets a certain percentage of the licensing fees as royalty. Most songwriters sign over publishing rights for a relatively paltry percentage. Some who are canny, or at the peak of their careers, command a good percentage, or contractually stipulate reversion of control at some point.
The Beatles set up their own publishing company (Northern Songs) in 1963, along with an existing publisher, Dick James (most famous as the owner of DJM, which made a fortune as Elton John's original record label).
In 1965, the owners decided to float Northern Songs, to save on capital gains tax. This left Lennon and McCartney each with 15% of their publishing rights. In 1968 they signed on for a further five years - the lucrative early years of their respective solo careers. However, their relationship with James deteriorated (as subsequently happened with James and Elton John), so in 1969 James sold off to ATV Publishing for a good profit, without giving Lennon and McCartney a chance to buy him out. Since ATV threatened to take a controlling share which they didn't manage to buy out, Lennon and McCartney sold their shares in 1969 for £3.5 million.
When ATV Publishing was up for sale in 1985, McCartney tried to get Lennon's widow to unite to buy it up, but Ono preferred it to go to Michael Jackson, to avoid ongoing bickering between her and McCartney. It has been noted that at this point McCartney could easily have bought out on his own. But he didn't, for whatever reason, and it has since been said with some justification that Jackson had a better eye for business than McCartney.
In 1995, Jackson sold 50% into a joint venture with Sony which, with other major catalogue, became the third biggest music publisher in the world.
Sony/ATV's Beatles rights are currently estimated to be worth $30-45m per year. I saw one estimate saying Jackson's assets were about $400m, debts being about $200m; however, that didn't seem to square with Jackson's estimated holding in Sony/ATV: $450m alone.
That is likely to end up on the capital markets, simply because it is more prized as a capital asset than for its sentimental - or creative - value.