Frankly, I find your argument that stores shouldn't host reviews because it puts YouTubers out of work to be the least compelling argument that there could possibly be.
Well, then it's a good thing I didn't say anything even remotely like that. Where on Earth did you come up with talking about Youtube reviewers? Not from me, that's for sure. I barely touch Youtube for anything, and if I do, it's probably muted so I don't have to hear their useless talking.
Finally, your suggestion that the only appropriate way for there to be reviews on sites that sell products is for said seller to partner with a third party is also restrictive in its own way. Which third party should be the "official" review site for Steam? Who decides? How would that not turn into a corrupt enterprise? I don't think the current system is perfect, but tossing out user reviews does not seem like a rational solution to that problem.
Sure, there's still a conflict, but it has a CHANCE of being responsible, and the review source would need to compete to maintain its status with Steam, but you can also have reviews feeding into other stores, like Uplay, Origin, GOG, Fanatical, etc. A good store might even connect with multiple reliable review sites, to merge the reviews and give their customers good information. If one store chose to stack the odds with a front for a corrupt review operation, people might just not bother as long as any stores are demonstrating there are better review sources (the same as how I go to Steam when shopping on sites with even poorer review systems). People might even get used to using their own sources directly, if we had a market where those sources could usefully exist.
What we have now is a system at best cobbled together haphazardly, poorly, and with no outside transparency at all. Companies can even drop a product to hide its reviews and then put it up in a new version with a clean slate, like Planetary Annihilation and other games have done. No one even tries to make reviews/ratings work better for the users, because they can't do anything when the stores control the reviews and people don't care how they let their social network participation kill the opportunity for competition. No one would expect this system to work, and it doesn't. We deal with it, because much like the rest of the things Steam shouldn't be doing, we don't have a lot of choice.
You wouldn't need the store to serve reviews to you if stores weren't already doing it--and keeping others from doing it better. It doesn't even matter if another store has reviews, unless the game isn't available on Steam, because people will go to Steam for the reviews anyway. It's a feature a new store can only look inadequate by attempting. Doing reviews better than Steam won't make people use them enough to produce the content needed to outweigh Steam, so it's basically wasted effort. Pretending it's a feature you expect them to compete on is disingenuous. More importantly, it ignores the lock Steam has by its corrupt requirement to buy via Steam. You're demanding a feature they can only waste effort doing, where they can only look bad by trying, and where even placing it as significant
only drives sales and traffic back to Steam.