why expensive streamers

(someone’s anecdotal experience does not constitute proof).

It does to them and that's all the proof they need.

Why are people spending 1000's on streamers?

We're idiots.  Obviously.  

Happy, happy idiots.

🤪

I think that we can all agree that the data is reaching the streamer and DAC intact. Networks are extremely reliable. But when it comes to audio, and reconstructing an analog signal from digital data, timing is very important. Good streamers use the best clocks, like OCXO, to achieve this for their SPDIF outputs. Good DACs use them for their USB inputs. This is just one factor in properly transporting and converting digital audio signals, it is quite different from displaying a simple data file on your computer. 
 

While I am not a strictly measurements based guy, I thiink you have to hear a component, and live with it for a while, before you can judge it. However Amir offers an excellent primer on digital measurements that gives you some insight into the complexities of digital audio conversion. You can read about it here:

Good streamers make a difference, it is that simple, and I don’t think you need a super high buck system to hear it. 

I suppose I'm the outlier here as I have tried many streamers and have yet to find one that's reasonably priced for what you get and that actually makes a sound difference that's worth the silly cost of some of them. Personally I have a dedicated micro PC that has been optimized for Audio streaming ( not an easy task) and up to now is the best sounding, it clearly beat the Aurender A10 I had and beat the Chord streamer I currently have (those both beat the Roon Nucleus I had prior). other then its a pain to use as you need a mouse, keyboard, TV/monitor.  I have found the DAC is by far the more important piece in the digital chain and its implemented will determine if your streamer is going to make much difference or not.  Well engineered DAC's are less susceptible to the streamer then poorly engineered DAC's. 

It sounds like what you are describing is a "server", not a "streamer", which often includes a DAC.  If you're expecting a server to vastly improve sound quality, you're correct in your assumption that it will have much less impact than the DAC will.

I went from a Roon Nucleus to a much more expensive server and was frankly somewhat disappointed that there wasn't much difference in sound quality, however there are good reasons to have a good server (which I consider the Nucleus to be) - onboard SSD storage, Roon compatibility, better connections to the DAC among them.  

In my case, the server I have also includes a top notch CD player that reads the CD into a buffer on the streamer, upsamples it, and sends it to my DAC.  It also has i2s connectivity and the two pieces are from the same manufacturer and designed to play well together.  One of my previous DACs was not ROON ready, but connecting it to the server allowed ROON to "see" it.

My opinion / experience is that for most people an expensive server will have little if any impact on sound quality, but may offer other benefits.