Because an "active preamp" cannot increase the dynamic range any more than what source is giving, unless it has an inbuilt "dynamic range enhancer" (DBX and they sound s**t). It can only serve to reduce the dynamic range, because it's not "a piece of wire"I don't like DBX either for the same reason. But that's not the problem; what a good preamp can do is not mess up the relationship of the output impedance of the source with respect to the cable or the amplifier input impedance. For this reason a buffer without gain can often work as well if no gain is required.
IOW, a preamp does not have to increase dynamic impact, it simply has to preserve it, which many passives simply fail to do.
Its only taken digital designers about 20-30 years to come around to the fact that they need to put a good quality volume control in their gear. Because so many don't, this is a good reason for an active preamp. Additionally, should any other source be desired, like a tuner or turntable, a preamp is mandatory since those source don't drive a power amp to full output.