If you played with your cards face up you could make a bet giving 4-1 on a flush draw and be correct.

However if you ever pay off a bet on river when flush card hits you will now be giving better than 4-1.

Similarly, if you always fold to any bet when a flush card hits on river you will fold a lot of winners, as you are easily bluffed.

Unless you are 100% opponent is on flush draw it's best to not try and give too tempting odds.