Which specific streamers have you actually auditioned to reach your conclusion? Are you able to hear differences between DACs? Have you perceived differences between amplifiers? To what might you attribute those different sonic characteristics? Why would that not also apply to a source?
It turns out they do. Though you may believe differently, with any reasonably decent system (golden ears and/or golden wallet are not needed) those differences are readily apparent. Yes, a cheap streamer will play music with fidelity. Better units will do so with more spatial information and resolution which are pretty easy to hear. Perhaps you should try one - you might be pleasantly surprised.
This is an audiophile forum - a place where people who are interested in bettering their sound (within their budget) come to discuss and explore. It's interesting how many come here with preconceived ideas of what "must" be (i.e. bits are bits so digital sources sound the same, all class D amplifiers are lifeless, etc.).
I'm not suggesting that more expensive is always better sounding. That clearly isn't the case. But, a good system will be well rewarded with moving upstream on the source. BTW - I didn't believe it either before I tried it. Nevertheless, I gained more sonic benefits moving to a better streamer than I did by significantly upgrading my DAC (and I spent 2x on the DAC relative to the streamer). I think I may have gotten that backwards in hindsight.