from flop to river

Hi,

why are flop decisions based on outs for the turn and not for the river?

For example: I'm HU with a nut flush draw against Toppair. It is bet to me and I'm considering calling.

Standard calculation:
My chances hitting my flush on the turn are 1-4.1 (9 outs). The pot has to be at least 4.1*0.5BB = ~2BB.

My calculation:
I can calulate what I have to pay to see the river. 1/2 BB on the flop and 1 BB on the turn e.g. 100% of the time. The total cost of seeing the river is 1.5 BB. My chance of hitting the flush by the river is 1-1.9. So the pot has to be at least 1.5 BB * 1.9 = 2.8 BB .

The latter calcuation doesn't seem correct because I don't consider an extra bet I could get either on the turn or the river.

But how can the first one be correct? It doesn't account for:
*extra bets I could get on the turn (or river)
*bets I'm likely to pay on the turn (or river)