why expensive streamers

The streamer is the first point at which timing (jitter) becomes a thing. It's job is to unpack the packets/frames of data which arrive asynchronously and convert them into a bitstream. An highly accurate clock at this point can have a huge impact on sound quality, as of course can the streamer not adding in unnecessary noise which won't make any difference to the 1s and 0s but if it reaches the analog(ue) parts of the DAC then it very much can.

I still only have a modest Innuos Zen Mini with a souped up Zen Mk3 LPSU but the improvement in sound quality over my Bluesound Node 2i is clearly audible, and that's despite the latter having been "pimped" with a Sean Jacobs DC3 linear PSU and a Mutec MC-3 reclocker.

This whole thread reminds me of the debate about amplification.  There are audiophiles that claim that if 2 amps measure the same then there is no sonic difference between them.

  I’ve tried several streamers in the $500-$4500 range and there are definitely differences. 

I don't claim to be the world's most sophisticated person (let alone audiophile) when it comes to digital audio and playing music from a cloud-based music service, or a hard drive/server.  I do know more than the average person about audio playback systems, acoustics and reverberation time, amplification, cabling etc.
What I have experienced firsthand is the sonic improvements that I have heard when moving source from a high-quality compact disc player, incorporating an outboard DAC (the initial offerings from Theta Digital, Wadia, etc.) and then moving onto a compute solution with an outboard compute-based DAC system. 
First a small Benchmark and then to a Lampizator Baltic 3.  Then USB from compute to the USB input of the Lampizator.  And the differences there were not subtle. 
Then along came a Lumin U2 and WOW! 
Can I technically explain why it sounds so much better?  Maybe a little.
But you know what else adds to the overall sound quality of these more expensive products?  It's still a lot of good quality and attentive detailed engineering.  These things still plug into a wall, have a power supply, have circuitry that are sensitive to resonance and heat, etc.  And there is still that pride of ownership.  Being able to look at something and appreciate what went into designing it, the build quality, chassis finish, the tubes that glow (in some cases) are also all parts of why we purchase what we do.
If that didn't exist then Rolex, Breitling, Phlip Patek etc. wouldn't be providing their watches in the marketplace.  I mean, don't all watches just tell time?  Why own an expensive one?
Same goes for cars; yeah, you can buy a car based on specifications alone, but specifications don't tell you what the leather smells like, how the door shuts so quietly, and how the engine sounds when you turn it on and how fun it feels when you take a corner at 70 miles per hour while still feeling so safe.  (These are just analogies so don't critique my driving habits, please!)
I know I am not helping the discussion specifically, just something I wanted to throw in there!
          

Streamers and dacs work in tandem. I wouldn't get the best out of a DCS Vivaldi DAC  using a Bluesound Node, just as I wouldn't get the best out of a Grimm using a Topping DAC.

Same goes for cars; yeah, you can buy a car based on specifications alone, but specifications don't tell you what the leather smells like, how the door shuts so quietly, and how the engine sounds when you turn it on and how fun it feels when you take a corner at 70 miles per hour while still feeling so safe.  (These are just analogies so don't critique my driving habits, please!)

Right the specs can only tell so much.  You have to test drive the car to know how it drives.