There's a mixture of both. The pension officer for institutions like California Retirement System will have his/her ticket paid but other folks that run their own small fund will pay their own way.
The attendees all have the problem of "where do I put my institution's money?" and they go to the conference to listen to various presentations to be persuaded. Yes, there are also chances for networking and "meet & greet" but the event is largely a marketplace hub for "sellers" (funds) and "buyers" (institutions). The sellers (e.g. the hedge funds and the billionaire managers that appear on the stage) sponsor the show which in effect also funds Milken's 501c charity institution.
It's in getting into a room of sales prospects and potential investors who have already demonstrated that they can afford to spend $12k on a conference. It's like deliberately going to a bar with a steep cover charge so you only have to mingle with the "right people".
The best events I’ve been to, with the most interesting and talented speakers, were always cheaper.
And your comment reminds me of one of my other examples of the "buyers buying ads":
It isn't just small subscription amounts where people buy the ads in magazines; sometimes people will pay $12000 to be bombarded with ads. Who are those people and why would they do this?!?
The example is the attendees to the Milken Institute conferences[0]. The ticket price is ~$12000 and it allows attendees to listen to "ad presentations" from fund managers. (Example.[1])
Yes, it's formally called a "discussion panel" but everybody knows that the fund managers on the stage are really advertising their firms' investing expertise without the vulgarity of explicitly labeling it "ads".
If one only has an uncompromising view that _all_ ads are evil, it will not make any sense why pension fund custodians would pay thousands to view ads. On the other hand, if there's wiggle room for "relevant ads", the payment of $12k for ad presentations is perfectly rational.
[0] https://www.milkeninstitute.org/events/conferences/
[1] https://www.youtube.com/watch?v=TpFOPHUhRBg