Because I'm not in it for social reasons. I got burned many years ago with another game that was entirely on a social level. I enjoy seeing what I can do by myself. I'm not out to win competitions or have the best decorated town. I don't want to be tied to a game and with social play IMO that is what happens. I'm 3 factories behind right now because I just haven't had the desire to build them. When I need those products for a plane, I buy them. I also have not spent any money buying TC. I've earned what I have by watching ads or completing in game tasks that reward you in TC. I use that TC to keep the dealer.
Like I said... I am a unicorn. I am not your typical player.