I think it’s a contraction of “When people are trying to obtain a particular song, they’d prefer to be able to obtain just that song, if they don’t also want the benefit or expense of obtaining the album in order to do so”.
This has been contracted into “People want to download singles”.
I can agree that whilst some peculiar people might like visiting the record shop or scouring the file sharing cloud, many would prefer that they could have instant access to any song ever recorded simply by thinking of it (or at most typing its name).
Even better would be for music, previously heard or not, to be automatically played to them according to what they’d most enjoy at any moment, without even thinking about it, and with such a service provided free of charge.
How are musicians to make money then?
Well, those who wish their favourite musicians to produce more music will pay them to do so.
Music should be a pleasure. So should making it, but those who obtain pleasure will be glad to reward the efforts of those who produce it – given it is not without effort.
The whole album vs single controversy results from the music=copy delusion (publishers looking at the bottom line in sales of copies).
One might have albums a la Pink Floyd that retain aesthetic integrity and some degree of continuity as a whole, or one can simply have an arbitrary collection of singles. The collection of singles has little life left except as a means of transporting the files. However, even in the future, the only thing the album as coherent work has against it is the longer attention span it requires to appreciate it.