It's not simulation. It measures all plays made- the ones hit right at someone, near them, or nearly impossible to get to, but some do.
Why would you think that player A is as good as player B, if they both make the identical plays hit at them or near them: same amount- same error rate, but player B gets to and touches 100 more balls over a season that are just plain out of the range of player A. He makes 80 of them and is charged with an error (glove or throw) on 20. You still think play A is better?
While this is a hypothetical scenario I am presenting to you, these situations are based in reality and can be proven by facts, This happens every year in MLB.
Simple observations can tell you some SSs get to balls hit very far away from them, or ones kinds far but hit sharply, while some just don't seem to make any or many more than 4. 5 or maybe 6-7 steps away, if the ball is hit slower.
Of course some players make plays others never dreamed they could make. How is that not a distinct value (or weakness, if you lack range?)
Sure, some speculation is involved, but when you figure errors are assigned or not based on subjectivity, then we also "speculate" it all evens out over time. Nothing is perfect- not fldg% or metrics.
Fldg% is just one part of defense. Thinking range and athleticism, such as being able to jump higher, run faster, or react quicker is not helpful or of value blows my mind away.
Thinking that some players position themselves better, read the pitch better, get better jumps and take the right angle to the ball better is not speculation or simulation: it is fact and it is observable with the naked eye and also with cameras and high tech data like ball speed, distance from defender, trajectory and more.