They're keeping them open because Sony needs a presence in London. All large publishers have an office or studio in the capital which usually serves the UK and wider Europe.
Eh, they're keeping them open because it's been a profitable studio for a long time, despite not generally releasing games you would like. (They are, however, overdue for a new hit.)
They're also keeping them open because the studio does a lot of support work and hardware incubation, stuff people don't know much about. For instance, a lot of original PS accessories and services were invented in or produced for release out of that office; you can track London Studio's work in physical add-ons for PlayStation from PS Eye to now PlayStation VR.
(Side-note, but I believe Sony's presence in the UK is mostly now run out of the Liverpool office, not London, though they do list their 10 Great Marlborough St. office as one of their corporate HQ addresses so maybe it's both? I wouldn't say you don't need a prestige office in a capitol to run a business in a country. San Mateo is of course pretty far from being the capitol of the United States.)
...I get that you look at London Studios' resume and see nothing you're into. And true, there's not been a lot in a
long time that says they're capable of a massive mainstream game. (Their last two attempts at mainline gamers, The Getaway PS3 and Eight Days, were both so long ago and both canceled in development; since then, the closest to "hardcore gamer" products have been VR titles.) Don't buy this fantasy co-op game if it doesn't look cool to you. It could possibly suck.
But just FYI, London Studio has been an important part of the PlayStation operation for a long, long time, sometimes in ways most gamers never become aware of. And they have people there who, for ages, have wanted to put London Studio back in the PlayStation spotlight.
(EDIT 2024 - The concept art attached up in the OP broke, so I'm rehosting it on GAF: )