Why are both main subjects usually heard in the tonic key during the recapitulation?

In sonata form, the recapitulation serves to reconcile the tonal conflicts established during the exposition.1

During the exposition, the movement introduces a dynamic contrast by presenting the first subject in the tonic key and the second subject in a contrasting key—typically the dominant key. This creates an opposition of tonalities that defines the movement's structural tension.12

The recapitulation is designed to resolve this tension. By presenting both main subjects in the tonic key, the composer brings the thematic material into a new state of equilibrium. This process allows the listener to perceive the subjects in a unified relationship, effectively "reconciling the opposites" that were set in motion at the beginning of the work. While composers may use various modifications or reshufflings of the original material to achieve this, the fundamental goal remains the same: to return the entire musical landscape to the home key, providing a sense of finality and synthesis.1