Adding stuff to a game has multiple reasons, the majority of reasons have nothing to do with game design, more likely with business.
In short: you cannot simply just keep “adding stuff” to a game and have it retain its solid fidelity. There is a point with every game where adding one more monster, one more gun, one more playable class is too much. This may not entirely break the game, but it does degrade it. The quality and significance of each decision is slightly diminished, the identity of elements blurred.
You can't compare games like chess with BF2/TF2, because chess is a game concept, a rule set, and not a product. To some degree you need to compare chess to a FPS or a RTS or a RPG and then you will see, that a FPS/RTS/RPG has not been greatly expanded since the first nethack,doom or warcraft game. Considering that games like chess evolved over 100 of years, a FPS/RTS/RPG game will be almost the same in 200 years, just the appearance changed, like a chess board from an other manufactory.
Take a look at half-life. This game was mostly a single-player game, but the real success was its multiplayer part due to great modding support. This game was one of the first one which demonstrated the power of solid game community. First, a multiplayer game is much more likely to generate a online community, secondly you need to feed your community to keep it at it. You know that gamers want to play, and that they will leave your game (more importantly your community) to join something else once the get bored. This is the main reason, publisher provided ongoing content: to keep the people in their game until they release the according sequel
It is just standard business, like a payback card from your discounter. They don't want to directly influence the amount of products you consume (that is markting), they want to influence from where you get your products(when you buy it, buy it from us).
It isn't a secred, the industry want your money, not your happieness. So, when you want to make games which makes people happy, don't expect to get rich (play lottory, it is more likely to have success with it).