You're overestimating a couple of things slightly - Winn didn't appear till the last ep of the first season, for example, and Dukat appeared in only 35 eps out of 175, although come to think of it that is precisely 1 in 5.
But I think the point is that these characters are so well developed and so much an important part of the continuing narrative that they feel like
they appear more frequently than they actually do. Garak feels as much of a main character as any of the others, and yet he only appears in 33 episodes.
That's the joy of the show - that the supporting cast make up a more interesting set of characters than the main cast of other shows. And that is thanks to something that seems like common sense but actually happens vanishingly rarely - writers looking at what was already established and asking "what happens next?" Aka, continuity.