why expensive streamers

I seriously doubt that the difference in streamers has anything to do with data integrity. I would expect that every streamer can deliver the digital information to the DAC without introducing any data errors. 

But DACs are very sensitive to analog noise and timing errors on the digital data signal. Higher-end DACs put a lot of effort into minimizing these effects by using digital isolators, reclocking, PLLs, etc. but these DACs also tend to be more resolving allowing differences in streamers to be more obvious. 

High-end streamers go to great pains to minimize noise and clock errors by utilizing complex data isolation and FIFO reclockers, extremely high-accuracy clocks with low jitter and phase noise, ultra-quiet power supplies (even battery or ultra-capacitor powered), high-bandwidth digital line drivers, etc. to deliver the cleanest possible signal to the DAC. This does not come cheap. 

How much this affects sound quality will, of course, depend on the DAC being used, how noisy the signal to the streamer is, the cables and connection type being used, and the resolving power of the overall system. 

But if I was spending $20K on a DAC, I certainly wouldn't expect to get the best out of it with a $500 streamer. 

What @jaytor said ^^above^^, and very well articulated. 

Lumin U1 with Aqua Acoustics LaScala DAC. SUPERB.

@jaytor thx for giving potential technical reasons for selecting one streamer over another, but I believe there is an easy solution for the issues you describe.WIth your assumption that streamers can get bits correctly to the DAC, all the DAC designer would need to do is put a simple memory between the input bits from the streamer and the rest of the DAC. This memory can be designed so each side has its own power supply and clocks. My understanding is that DAC analog noise and timing errors occur in the digital processing and conversion to analog circuitry. The memory would assure that the analog noise and timing errors from the server would not affect this DAC circuitry.

The memory would assure that the analog noise and timing errors from the server would not affect this DAC circuitry.

That is not quite correct.  The buffer memory on the DAC is there to ensure there is no underflow - that is the data will always be there.  The memory buffer is not intended to isolate the noise.  The noise from the streamer can potentially affect the DAC timing.  This is true for asynchronous USB. The noise on the ground and supply of the streamer can inject directly to the DAC circuitry.  That is why some DAC uses optical isolation which minimize the coupling hence reduce the noise.

For S/Pdif it is a little bit different but in this case instead of a streamer, you have the transport noise affecting the DAC.