The people who put their artistic efforts into creating a video game want their art to be appreciated by as many people as possible. True, the game is a product which is meant to sell, but so is a significant percentage of art. Like the video game creator, an artist may also choose to mass-produce their art because 1) it will make more of a profit and 2) it will reach and be appreciated by more people, as opposed to sitting forgotten in the storage of an art collector.
Being mass-produced and sold as a product does not invalidate an artwork as a work of art; an artist does indeed want to express themself, but needs to make a living, too.
The creators of video games are the same. With all of the time, effort, tech, and budget that is put into creating a video game, a profit needs to be turned in order to compensate the artists and programmers for their work. This does not mean that the creators of a video game did not intend to express themselves artistically; there may be a deep, underlying message in the game's story, or an aesthetic purpose behind the chosen art style. Why use cell-shading as opposed to realism? What story are the creators trying to tell? What feeling or atmosphere are the creators trying to convey?
Art is self-expression. It is also a means of living for many artists. Video game creators are no different than artists of other mediums in this regard.