Everyone has their own preferences and I respect yours. May have something to do with the differences in age or how we were brought up. I grew up long before home computers when games were cards and board games. I used to play the war games by Avalon Hill and SSI. These were hardcore board games with giant boards about 3-feet square and hundreds of hexes on the board. A single game with two players often took several days to complete, that is, if you could keep the board from getting turned over during this time. When computers came and RTS games appeared, I thought it was great. Now I could play these games in real-time, not turn-based like board games. It's the reason I despise turn-based strategy now (with the exception of Moonbase Commander). And those game I could play single-player, but sandbox MMOs just don't offer the same challenges for SP mode and I see them more as social games.
Aside from this, games should offer both options, I'm just saying that MP is more important and should be priority in development. What SP computer games do you know of that are still popular after 10 years or so?