It's because console games are traditionally higher priced, and I guess they don't want to be undercutting their faithful console gamers?
Plus if it was cheaper - Wouldn't it have to be cheaper all around, as buying one gets you access to both PC and Xbox One version now.
So savvy console gamers would just all buy the cheaper PC version to play on their consoles.
You would think they could just price it £10 higher in shops than for a digital download though, as most consumers would understand the cost of pressing a disc, manual, producing a game case and shipping them out to stores is considerably more than storing it on a cloud server for people to download.