Because they are classic French antique mirrors. And because we love to pronounce the word- trumeau- slip it into the conversation and it will have the same effect as supercalifragilisticexpialidocious- it will say it all.
The word itself describes the space between the windows or doors, but it is used nowadays to describe a pier mirror/ above the mantle mirror. If you're lucky enough to have the space for a trumeau mirror in between two large doors spposing the windows, than that will be precisely the place for them- they will bring both light and garden indoors and serve as an objet d'art. With beautiful painting and/or carvings on the top part and sides, if original, they would have been part of the boiserie (paneling) of a room as was the fashion of the great houses of the 18th century.
They are now back in fashion and we all want a piece of it- they are a great talking point and, hung with thought and style, a trumeau mirror can be a true luxury item.