Because London isn't the be all and end all.
Perversely, rural areas may turn out to have a better business case as Openreach isn't likely to face competition. Like you've said yourself, the likes of Hyperoptic will try to swoop in with a cheap offer and get the landlords / building management on their side, and make it difficult for others to come in even if they wanted to.
In smaller towns and rural areas, they can do everything themselves and the only permission they will need is from the end user. Additionally, they'll have much less red tape and the local councils will be far more appreciative
Those "Mayfair hedge funds" and other "prosperous businesses" don't need consumer grade FTTP - they can have pretty much anything they want, from the umpteen companies who have already dug up every street in Central London. Including Openreach