Why does mixing more pigments tend to make a colour mixture darker?

Mixing pigments results in darker colours because pigment mixing is a subtractive process.12

Unlike additive mixing, which combines beams of light to create brighter colours, pigments function by absorbing specific parts of the visible spectrum while reflecting others. Each pigment added to a mixture introduces its own set of absorption characteristics. As you combine more pigments, you increase the number of spectral components being absorbed, leaving less light to be reflected to the eye.12

If enough pigments are combined, they can absorb nearly all incoming light, resulting in a dark or black appearance. For example, while a single pigment might reflect a specific range of light, mixing it with another pigment that absorbs those reflected wavelengths will effectively remove that colour from the final mixture.12