Reminds me of story where (and I'm going to butcher parts of this) a town is making a huge vat of wine (or maybe it's food related) and everyone household is supposed to bring a bottle to pour in. A couple of people decide to cheat the system and bring water instead expecting to hide under the radar. It turns out way more people than a few take this path and so the "wine" just tastes like water with a hint of wine in it.
As it's Amazon, presumably they tested and found this to be the best mode for profit.
Is it really identical? They do have quite a bit of warehouse which means that shipping can be quite longer and more expensive.
Let say I'm a third party and I got 100 shirts in Amazon warehouses in north America and they got 100 too. That means that in average there's 2.6 shirts per warehouse (I found there is 75 warehouses but that can be a wrong number) but we each got 1.3 in averages! If someone buy 2 of mine, most likely they'll have to ship one from somewhere else, while they could just take one of theirs and consider it the same.
At the end of the day though, it doesn't means that I was truthful and my product is actually the same, which can hurt their orders.
There is a separate review system for sellers, and the score is shown when you look at the buying options, but it's little used and much less visible.
It's a bit fraudulent really, like a bait and switch.
In reality each page needs a product and supplier rating, and people need to be guided to rate both. Instead you get "this company sacked my daughter; 1 star" and "the postman dropped it; 1 star" type ratings.