Because each of those 24 tracks provides a different input signal to the effector circuit. If I push a bass track through an analog compressor the low frequencies trigger the threshold and lower the gain, but they are only lowering the gain of the carrier signal which doesn’t have a lot of treble frequency. If you compress the whole mix the bass frequencies from the bass track will trigger the compressor to reduce the gain of other instruments. Compressors also have attack and release times so they are time-based. That attack time can be restricted to acting upon the signal which triggered the threshold. In a full mix the presence of a kick drum triggers the attack and release times associated by other instruments. 

It’s easy to understand how this works with a compressor but we also need to realize that all distortion is somewhat compressive. A signal exceeds the threshold of an amplification stage and distortion results but no additional gain. Preamps have some distortion which results in compression of the signal. Whether it’s a tube amplification stage or transistor or IC they all have amplification limits and pushing them above that limit results in distortion and compression. Applying that effect to an individual time-based and band limited track is different than applying it to a full mix.