I can't tell you about scientific reasoning, but as someone who spends a lot of time browsing perfume sites, my guess is that it's to emphasize the bottle and perfume.
The design of the bottle is as much a selling factor as what the perfume smells like. You may have differences among lines, but generally, a brand has a look, and a family, so that you know what you're picking up before you even smell it. (Perfume makers also have reputations and brands, so if you like one scent by Olivia Giacobetti you may like others, but that's not a design issue.)
Examples of brand lines:
The Different Company
Boadicea the Victorious
The color of the bottle, the label, the liquid itself are all part of the selling experience. Marketing copy emphasizes how the thick solid bottles of TDC feel good in your hand and are refillable, while Comme des Garçons has smaller, daintier bottles because their product costs a third of the price of TDC's.
A minimalist monochrome website design emphasizes the product. I'm thinking that several companies figured this out independently, and the rest followed suit because it worked.
(I note with amusement that Lucky Scent, the website for all those links, does not have a monochrome scheme.)