The answer: they’ve never tested it.
“In the past, you must have come across one of those long sales pages that never seem to end. Most designers hate long sales letters because they contain heaps of text but apparently lack on aesthetic fronts. Some people go to extremes and even call such sales pages evil forms of internet marketing. Truth be told: long sales letter pages are based on learnings from direct marketing and the catalog industry, so there is nothing inherently evil about them. In fact, many marketers still use long form of sales pages because they convert visitors like crazy.”
37signals’ long sales letter page increased signups by 37.5%