Why so many long books?

A loyal MR reader writes:

Why are there so many well-padded books out there that really ought to be nice, long articles?

David Sucher has raised similar questions in the MR comments.  The answer is simple: most people don’t read the books they buy.  But they like the self-image generated by the book purchase decision, and they like to feel they are getting something for their money.  Driven by market demand, book publishers demand a certain amount of heft and sometimes this means padding.

Yes there is a tendency toward shorter "books," some of which are called blogs.  The price is lower.  Another loyal MR reader once wrote in praise of MR: "if I wanted to read something longer I would read a book or something".  Or not read, as the case may be.

Addendum: Note also that marketing expenditures are more or less constant, relative to the size of the book.  Higher marketing expenditures (definitely the trend) thus spur higher-margin and typically larger books, as suggested by the Alchian and Allen theorem (why buy a big ad campaign for book which sells for a penny?).  For those of us who actually read the books, as book choice goes up, the importance of marketing goes up, and the padding goes up as well.