I disagree with the implicit belief that every game should be fun. There are other metrics to judge games by. Though it might be needed in this particular game, it's not true of all games as a whole, nor of every mechanic in games.

The Beginner's Guide did not need fun to keep me, at least, engaged. Dwarf Fortress's fun must be made almost entirely by the player. Boring or downright hostile mechanics - such as long travel times in X3, or the long slog of rebuilding an army after a disastrous war in Mount and Blade, or permadeath in Darkest Dungeon and roguelikes - are not fun, yet I contend that without them the rest of the game would feel boring, even weightless. The unfun parts don't just "pad out" the game, they give meaning to the player's decisions and make choices matter.