I've bought my fair share of tickets off of StubHub but I've only sold there once and I don't remember the transfer process because it was 4 years ago.
My initial take would be that they would do this either:
a) to act as an intermediary (themselves) that you transfer to and then they transfer to the buyer's Ticketmaster account, so that you don't get someone's PII
b) they buy a few near face value to have when inevitably, fake tickets are sold and they have to replace them ASAP (which has happened to me before -- as in, I was at the venue and couldn't scan one of my tickets and then went in and called StubHub and they called me back and got a backup ticket for my friend)
But my gut tells me it is probably the first? This would also cut down on customers telling them they didn't get a ticket transfer (if they really did) and also ensuring that sellers actually transfer tickets. An easier method of all of this would be to have a way to work directly with Ticketmaster or AXS or whoever to act as an escrow agent for ticket sales, but considering AXS and Ticketmaster have their own resale services, I'm sure they make that as difficult as possible.
I have to think that the biggest ongoing costs to an operation like StubHub or Seatgeek or Vivid or any of the other marketplaces are fraud and customer service related to lost ticket transfers. So anything you could do to mitigate both of those (which acting as an intermediary would help with both) would be worth it.
Siderant:
The thing that kills me about the resale/scalper nonsense is that in response, some venues are now doing anything they can to limit resales at all -- requiring you to pick up tickets from will call and show ID and credit card used to pay. So then you have to hope that whoever sold their extras on Craigslist or FB actually shows up at the venue so you can get your GA seat you paid way too much for, or else you're SOL. And like, look, I get it, you want to cut down on scalping. But things come up and people can't always attend shows. Or some of us will literally pay stupid prices to see someone live -- but when the venues put in onerous terms and check ID at the pickup window, that's just stupid.