How can they even find the average chance of scoring when there are so many factors playing in?

Where is the player on the pitch? From which direction did the pass come from, and at what speed? Was it a high pass, low, did the ball bounce or change direction on the way? Where are are the defenders positioned when the player is making the shot? Is the pitch dry/bumpy/wet? Just some random examples of the million factors playing in.

Which factors matter and which do they ignore? I don’t think they have good enough data when considering how “unique” most goal attempts actually are, all factors considered.