Virgin actually have upgraded a huge swath of their footprint from cable to XGS-PON (probably coming up to 10million homes now, with the full program due to finish in a couple of years).
However, due to their comically bad billing systems (i believe they licensed a billing system off the cable modem headend provider) they do not allow their existing users to switch from DOCSIS cable to FTTH. This has been a problem for a couple of years now. They've spent billions on civil engineering work to blow fibre everywhere but existing customers can't order it because their billing system is tightly coupled to their cable modem system. They offer up to 2gig symmetrical over XGSPON FTTH.
Re openreach I think it's a bit of protecting leased line revenue, a bit of faster upload speeds actually being quite niche - the market is driven by headline download speeds - but most importantly they rolled out GPON not XGSPON.
GPON "only" has 2.5gbit/1.2gbit available to the entire network slice it's on, which can be up to 32 homes (theoretically many more but openreach have that as the maximum I've seen).
This means one gigabit uplink can nearly saturate the entire link for the network slice of 32 homes.
They do have plans to upgrade to XGSPON (though I suspect they may skip that and move to 50GPON instead). XGSPON has 10git/10gbit and 50GPON 50/50 available to the same 32 homes.
They are just about to start a pilot of XGSPON in Guildford which has up to 8gig symmetrical available.
It's not a huge amount of work to upgrade PON versions, it just requires new line cards, and new ONT boxes for each house and can run side by side with existing GPON.