If the entire bass was made of a very rigid material, when you plucked the string, the vibration, or the sound, would begin and continue at a fairly high volume for a fairly long time. Because the bass would be rigid, all of the energy of the pluck would go into vibrating the string, and the string would slowly stop vibrating only due to the friction of the air and the string itself.
If components of the bass was made out of materials that were less rigid, and had joints that were less rigid, there would be friction in the joints and in the material itself that would take energy away from the vibration of the string and the vibration, or the sound, would fade away quicker.
Passive pickups only "pickup" the vibration of the string as influenced not only by the pluck itself, but of how the string vibrates within the system that includes the bass materials and joint construciton.
An active pickup has more going on inside of it and will not be as influenced by differences in the bass materials and construction. Please note I said "will not be influenced as much".
So we blend different materials together to create a bass that vibrates in a way that we personally like. Some people like a brighter sound with more sustain. Some pepople like a darker sound with less sustain.
So we look for, or should I say constantly look for, the perfect mix of materials and pickups and preamps and strings to find the tone that we personally like today. And then we hear something different and want to change the way our bass sounds. So we change out pickups and preamps and strings, and then go buy a different bass.