Pot odds and Calculation of outs

Please forgive the total ignorance of the question...being a fairly seasoned player, BUT, assume 10 payers, nlhe, and it goes heads up with one on the flush draw after the turn (four flush). How is it we use 52-4(flop+turn)-2(hole cards)=46 cards with 9(left of suit)/46 as the outs for the flush on the river, when 16 other cards, (eight players x two hole cards) have folded. Even with random distribution of suits, why do we not discount for these cards?

Because there's random distribution...

Put it another way, say I had 52 cards, the chance that the top card is a heart is 1:4, agreed? Now if I take the bottom 10 cards from that deck and hide them behind my back, does the probability of the top card being a heart change? No, those other cards are still part of the total sample as long as they remain unknown.

Yep. Those folded cards are just as unknown as the cards remaining in the deck. We don't know anything about them so we have to count them as part of the deck.

The only time this would change is if we somehow knew something about what cards were folded.