Because every chord has (usually) a root note which stands out more than the others. If you are humming something based from only chords then you are probably humming the root notes.
When there is melody and chords togethere the melody is often in a higher register than the chords, or otherwise it is written in a way that is more pronunced and easily stands out from the rest of whats going on in the music. The melody is the most important aspect of the music, everything else is made to support the melody. Rhis will make you as a listener easily able to pick out the melody from the context.
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