A flush is surely more rare than a straight; I think the problem is thinking about it counter-intuitively. Flushes are tough because there are only 13 cards of each suit and you need five of them for a flush. However, a straight can "overlap" 5-high straight with Ace,2,3,4,5 but then also a 6-high straight is possible via 6,5,4,3,2. This "overlap" works all the way up through King and then back to Ace again because A, K, Q, J, T (10) is a straight. Also, note that suits are irrelevant for straights. Take the 6-high straight example from above. Each number card could be any combination of suits.
When you approach the hands this way, then you begin to realize that a straight isn't as rare to get as a flush.