Having tenants requires work, adds cost, and might just not be worth your time.
You might just want a house for a weekend, and not want plebs messing your shit up.
And it probably has some degree of truth, You can stand outside 'new' buildings for a week and never see lights on in some windows.
I'd guess the price of these is around £350k - £1.5m (for a reasonable 1 - 3 bed flat in a new build), which is well within the reach of well off foreign middle class investors.
Incidentally, I just had lunch with an architect and a guy who runs a building company, both London-based. I asked them about this. They both agreed that no sane investor would leave flats bought as investments empty. Investment means basically having tenants. The agreed that there is a media-driven moral panic about this. The architect said he had one client who had a 12 bed-room house in central London that was empty for most of the year ... that customer was a member of Kuwait royal family.
I'm not saying such speculative investment doesn't exist, I just doubt it is as ubiquitous a phenomenon as claimed.