Unfortunately, I'm sure it's also done to save costs. Not just the cost of a hardcopy (if included), but also the cost of labor to create one. I guess some feel that the fans will come up with a wiki anyway, so let them do the work and the dev can save money.

Dwarf Fortress has no manual. It takes a good month to learn that game.

Tutorials are fine, but they should be supplemented - in complex games - with some sort of manual or printable reference card. It need not be a Falcon 4-esque 500+ page monstrosity. But it should cover the basics in the event that you, for instance, put the game aside for a while and have forgotten the basic mechanics by the time you return to it, or if you simply forgot something you learned earlier in the tutorial. 2-4 pages would suffice for most titles I've encountered, and that PDF can't cost very much at all. Slap in some already-generated artwork, throw in a key command chart (leave space for the player to write in their modified layout), and maybe explain crafting / spell mechanics and other stuff like that.

Aurora. Many gamers compare it to DF.

This.

If a game has a combination of an intuitive GUI, in-game explanations (e.g. tooltips in the inventory) and extensive tutorials, it's perfectly possible to go without reading a single page on paper or in a PDF reader. And this doesn't even depend on the genre and can be valid even for complex games (let's say RollerCoaster Tycoon 3 for example).

Well, try to play a game of the Civilization series without the manual and you`re lost! Well, you can peek into the civilopedia, but it doesn`t provide all necessary information.
I still own the boxed version of Civilization II, it contains a nearly 200 pages thick printed manual which explains everything you have to know about this game. The civilopedia goes then more indepth and explains more precise the civilization advances, technologies, religions, units and buildings.
Sure, for a simple shooter there might not be the need for a manual, but the more complex a game is, the more it needs a proper manual, where you can read everything about game mechanics and other stuff!

This must be why I always lose patience trying to play (and understand) Civilization games. Having to read a 200-page manual in order to play a game just isn't my thing, really.

I prefer games which you learn by playing. RTS games usually seem to be constructed this way: the missions start with simple objectives teaching you how to play the game and what it is about, but it increasingly gets more complex with each mission, you are introduced to new units and buildings etc. You don't have to know it all from the beginning, you learn as you play.

However, I recall the first Civilization game to claim to have a tutorial, but I guess it merely scratched the surface what the game was about.

I've got an idea : why don't Introversion compose a manual for Prison Architect and then sell it as DLC ? I'll would roll on the floor laughing if they did that, and I'd probably buy it as well, mug that I am !

I don't buy many new games, but I have found the last few (Metal Gear Solid V, Dragon Age Inquisition) have opted for the, 'download manual at our website.'
No Thank You.
I am not paying extra in printer ink for your manual. I want something physical and nicely bound to read.
But then I am from a time when the box contents were as exciting as the game itself. (Shadow of the Beast, Infocom games, Magnetic Scrolls games)

I got the game on Steam and if you click on manual on the store page you will get to the Prison Architect wiki (with a quick starter guide, tips, ....).
Link: http://devwiki.introversion.co.uk/pa/
For such complex games I prefer a wiki over a simple manual. Also you can find a lot of guides for this game (in all kind of languages) on the game selling site with "S". So perhaps GOG should at least add the link to the Prison Architect wiki.

P.S.: I read the part that you like to have an offline manual. It is just my opinion on this topic. ;) I don't know your game but I prefer a short tutorial over a manual. But you should be able to skip it (if you play the game again) and the tutorial shouldn't be half of the real game.

Xenonauts was overwhelming, only played 10 minutes and quit. :l

I think I'll make a video as an ingame tutorial would take longer :D. It's this game.

Thanks for the link. Too bad I'm not really a big fan of space games (exception: some Star Wars games). Otherwise I would try your game. Have fun with developing your game!

I take your point about how wonderful game wikis are but really that should just be in addition to a manual made by the developers. I want a game guide which is in one place and which I can access at any time, WITHOUT having to go online every time to access it. Then, after that, I can avail myself of the wonderful game wiki.

One of the worst (or best ?) illusrations of the point 'm trying to make is Crusader Kings 2. The manual for this game is about as useful as a chocolate teapot. Not only does it not introduce you to the rules, controls, interface etc., it also assumes that you are already an expert on the subject of feudalism. I had to work SO HARD to get even an oily grip on this game. And then, was it worth the effort ? Well, there are better games, let's put it that way.