While I don't know all the specifics of how the stats are computed, I do know it's certainly possible they are more encompassing than tour original assertion. This can be accomplished through even simple means, such as comparing runs allowed to outs recorded and attributing a fractional run to each put out. While this can't be boiled down to each individual at bat, what statistic can? Dustin Pedroia batted .318 last year, but I don't remember a single at bat where he received 318/1000 of a hit.
Statistics ate there to record the past. And this is true of all statistics, not just baseball. That they can be used to predict the fufuture is not meant to be so precise. And while it is true that no play exists in a vacuum, this doesn't negate the past. But boiling it down to a single at bat or play and saying statistics don't cover this is a gross misuse of the large scale data and the ultimate in small sample size arguments. No statistic anywhere is meant to be accurate or exact in that type of scenario, whether your talking baseball or weather or counting cards in LasVegas..