Christian video games are lame because Christian artists are bad.
Consider what makes a piece of art interesting in general. In mediums that involve storytelling you generally you have some kind of conflict that grows to a resolution. The nature of the conflict and the resolution gives a focus and clarity to certain aspects of real life. But, here's the big thing, generally speaking you don't write your story to be obviously about what the actual topic of the story is. You hide your meaning in at least one layer of symbols. And maybe you don't have any one meaning in mind at all, but you just come up with a rich brew of symbols that can be interpreted in many ways by different people.
Example: Anon above interprets Dark Souls in a way that the Lords of Cinder are Messiah figures. I interpret it in a different sense: the dark is the void, the cloud of unknowing behind which God hides. Both of us give Christian-ish interpretations to the story but they are different. In the end the story itself probably doesn't definitely intend either of these interpretations, but that's not a problem, because it permits them.
Now ask yourself how a Christian story can symbolize something beyond its obvious meaning?
Here's an example: the Creation of Adam. God's finger is stiff, Adam's is limp and impotent. The two fingers almost touch but not quite. There's a loving gaze between the two, but also a distance. The picture literally depicts the Creation of Adam, but its meaning is that man is less than God and dependent on God, that God always acts first to come to man, but yet that man still has an inestimable dignity. You could even interpret this in a non-Christian way if you wanted to, by substituting something else for God, say, reason. You could say "Man is almost reasonable, but not quite, yet perfect reason is the goal toward which we must strive," and that could be an interpretation that the work itself permits.
When Christians set out deliberately to make "X but Christian!!!" they generally drop all subtlety, all nuance. The point of the story they tell is "Christianity is true!" AND the content of the story is also "Christianity is true!". The content of their art SYMBOLIZES nothing outside itself. It is blatant and "preachy". It admits of no alternative interpretations. It does not allow anyone to draw their own conclusions. That's the reason why it's lame.
I'd much rather draw my own Christian conclusion about the meaning of a secular work of art than have someone else's Christian conclusion of a Christian work of art forced on me.