[0] https://www.independent.co.uk/life-style/female-bodyguard-hi...
[0] https://www.independent.co.uk/life-style/female-bodyguard-hi...
If you write for the Daily Mail then they also become 'virgin' for added sensationalism.
1. They aren't perceived as such, rather they can be mistaken for a partner, friend, or personal assistant. They are therefore allowed into places where bodyguards normally aren't and the attacker doesn't expect them.
2. Some rich prefer a female to guard their daughter or wife so there are no awkward situations.
(Quotes from the article above)
A unrelated woman is going to fit more discretely into a family situation then some ex-military/ex-police dude or other tough situations.
In some situations, I would imagine that in the case of a real threat that you'd have a layer of visible "beefy" security for prestige and random crazy person purposes, and a more subtle layer for other threats.