I know the reason to not include one is entirely for the publisher to save a buck.
You don't think it's any convenience for the consumer? I mean, if the manual is in the game, and if it can be accessed at any time in the game, then it saves you the trouble of having to get up and go flipping through the manual. This is especially useful to a portable system, where you may be out somewhere and don't have the manual with you
Secondly, this essentially allows unlimited space for the information in the manual. Manuals were probably getting less detailed because it cost less to print fewer pages, and with manuals today including multiple languages, that's still a lot of pages. But now, without cost restrictions for length, manuals can contain a lot more information in as many languages as they feel like translating without having to pack in big, bulky, expensive manuals. Now, if they don't actually start doing this then that's something to complain about... but hopefully they will improve them.