1. The simplest answer is: More gaming choices. Why just play multiplayer games?
There is a wealth of games out there for solo play, of all sorts of themes and complexity, including free print-and-play games.
2. Some gamers do not have a regular gaming group they can play with
They might live in a small town where it is hard to form a game group, or they may have a busy home life and cannot attend a weekly game night. For some their only gaming partner is their spouse or their children, and they don’t always want to play when you want to play.
3. Then there are gamers who work with people all day, so they find playing solo is more relaxing.
Some players just need some alone time, and introverts can be more themselves playing solo games. Playing a solitaire game means not having to teach the rules to others, and there is no needless distraction of arguments about rules, cell phone breaks, pack up time, etc.
4. Solitaire games tend to have more of a puzzle aspect and there are gamers who really enjoy tackling these puzzles.
They see it as an enjoyable intellectual exercise. They like seeing the way a designer recreates an opponent in AI, like the deck of cards in D-day at Omaha Beach.
5. When you are playing a solo game you can go at your own pace.
You can stop and read the rules. Take your time to think about your strategy. You can get AP without pressure from other people to hurry up--for 3 days. Some players like having a reset button. (Oops, screwed that up. Let's just start the whole game over.) Try doing that during your next multi-player game.
6. Another big plus for Solitaire games is that you can play games that your friends don’t enjoy.
They could be war games or history games. Some players just aren’t interested in recreating actual battles from the Napoleonic Wars. They could have long play times or more complicated rules and mechanics. Solitaire gaming allows a gamer to get more plays out of the games in their collection. They can buy a game they will be able to play instead of waiting for group to play it. They just can’t justify playing $50 - $60 for a game that only gets played twice a year
7. So what about video games? Why wouldn’t you play Assassin's Creed 4 instead of setting up a game by yourself on the kitchen table? Or how about watching TV or reading a book?
Well, if you are like me then your job is spent all day in front of a computer, so some people don’t want to spend their leisure time in front of screen. Also you don’t need to learn tons of button combos or have fast reflexes to play a board game. Watching a movie is a passive activity, and books do not allow interaction with the medium--you can never change War and Peace. Solo gamers like the tactile feel of board games: They like pushing counters, flipping cards and tossing dice. They can exercise their minds more playing solo games.
8. Some players like to play multiplayer games solo so they can better learn the rules and strategies.
Gamers can dissect the strategy aspects of a game and then play each character to test each facet of it. They can learn to play a new game before they have to teach it. One player said he likes to improve his multiplayer game scores.
9. There are many solitaire games that are story driven and feel more immersive.
When a gamer plays a solitaire board game, they get more of a sense of the story of the game. Many solitaire games are character driven, and you can become that character. There is an improved immersion because there is no one else to break the atmosphere the game--and you--create. You can talk to yourself, your characters, and tell the story as you play. One player mentioned they write up stories based upon their gaming sessions, and by playing solo they get even more story out of each session.