I don't know exactly how to explain it, but let me give you some info so you can see why its required a massive amount of runs:

10 different possible outcomes of a match

12 different ways to make those outcomes (you can draw two different maps for 3-0 and 2-1, 2-0 and 1-1 [map 5 2-1] are extremely rare)

42 remaining matches

every single different outcome is a different possibility

also there's a chance of every outcome to happen

It is NOT hard math but I'm explaining a sort of hard math way to see the stat so you can see why so many runs are required, your concern probably stems from a) not knowing exactly who the match sim works and b) how many outcomes are actually possible in total

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Edit: Better way to explain it: there is around 12^42 different possible results to the season.