why woudl openreach want to invest in an area where these is already blanket virgin coverage
To get users back on their network who currently use Virgin.
In most non Virgin areas there's less incentive for OpenReach to roll out FTTC or FTTP as they effectively have a monopoly over the area.
In Virgin areas with poor OpenReach availability a large proportion of users will be on Virgin.
Bringing SuperFast/Ultrafast will encourage customers back to their network.
OpenReach could possibly even get a better return on investment in Virgin areas as they make more from new customers they didn't have than they do from upselling existing customers.
There's plenty examples of OpenReach actively targeting Virgins Project Lightning areas with FTTP.
Within a few months of Virgin rolling out to my area OpenReach have come along and installed FTTP.