It would boil down to, bcos the design of streamer is more focused on streaming. And less of DAC functionality.
Second, compared to external dac which has a less complex circuitry, a streamer has just got too much of circuit , and that too handling complex software for streaming purpose, just like a mini computer. This creates more current noise, rfi etc.... which directly would impact circuit of the dac.
Third is common power supply and mix of above point. If by design there is single transformer handling duties of both streamer and dac functionality, it is bound to impact. This point may be technically wrong as i am no expert on it, but i still feel that way.