Is it possible that your mind is tricking you? Intuition in poker is usually wrong.
There are a couple of ways you could be mistaken.
1. Some of these games have you look at some cards and then choose whether you want to keep playing or not.
It could be that you are folding (choosing not to play) more possible straights than possible flushes. Remember that many of the possible straights are "inside straights" meaning missing a card in the middle rather than at the ends. Remember that the odds we calculated only apply starting from before any cards are dealt (once you see cards, the odds all change).
2. And, of course, you could just be being misled by your expectations. Your intuition is designed to look for patterns, and quite often it finds a bogus pattern. Once your intuition locks itself into a mathematically incorrect pattern, it is difficult to see the correct one.
Of course, it is possible that the game is fixed (i.e. not truly random). You would have to do an experiment to determine if this is true or not
If you want to do the experiment, you need to take your intuition out of the picture. The way I would suggest is the following.
1. Play the game 100 times (which is pretty good considering the hypothesis).
2. Never fold. Play every hand to its end.
3. Count the number of straights and flushes under these conditions.
If under these conditions there are the same number of straights and flushes, I would strongly suspect the game is fixed.