Why Are Chopin’s Preludes Not Introductions to Other Works?

The word “prelude” originally suggested an opening, as though a larger work ought to follow. Chopin changed that relationship in his 24 Preludes, Op. 28. Each piece is brief—some last less than a minute—but they are not unfinished fragments. Each creates its own mood, tension and closure.

The 24 pieces move through paired major and minor keys around the circle of fifths. A single prelude can therefore stand alone, while the complete set also has a deliberately shaped continuity. In the familiar Prelude No. 4 in E minor, the right-hand melody is restrained; the real movement comes from the left-hand harmony as it repeatedly descends and changes. The piece does not wait for another work to answer it. Within its short span, it moves from pressure and resistance towards stillness.

In performance, brevity should not make a prelude sound like a warm-up, nor should exaggerated tempo and dynamics be used to make it artificially “large”. The more important task is to hear each piece's central action—where the harmony moves, how the texture changes, and why the ending arrives at that particular moment. Chopin retained the prelude's sense of openness, but made that openness a complete form.

https://chopin.nifc.pl/en/chopin/gatunki/12_preludia
https://imslp.org/wiki/Preludes%2C_Op.28_%28Chopin%2C_Fr%C3%A9d%C3%A9ric%29