The reason for this is that Google understands exactly what service it's providing, namely the best search engine, and anything that's superfluous to that mission has been cut away so only the essentials remain.
The same principle can be applied to a photographic portrait: what's essential is the expression of the human face, a look in the eye, a pose, the line, the light. And all of these essentials are shown in a black and white image, with extraneous information like the colour of a the sky or a shirt cut away.
Of course, when it comes to something like photography no rule is writ in stone, and lots of time colour is not just appropriate but fantastic. So while many people do like the 'classic' black and white or sepia look I always provide a choice so you can decide what suits best.
