1: Because it does lead to "3e ish" desire from the PCs to buy their own magical items.
1a: The existence of a price list is a rational conclusion that a magical item market (shops, regular auctions, guild mandated price, something) exist. Otherwise, how were the prices determined, in world? Value is determined by something, and that something is usually the market. I'm in a home right now. I could give you every detail of the home itself, and you would not be able to tell me its value... until you knew where the home was, then you could look up the market and see how much similar homes are being sold for in this part of town.
1b: Therefore, players will rationally conclude that such conditions exist.
1c: However, it is highly questionable that the existence of such a market does make sense. After all... who's paying for these magical items? Who keeps a price list?
How much is a jade statue from the Ming Dynasty of China worth? There is probably an exact price, and a way to determine it. Just like we know that a gold cup set with emeralds is worth 7,500 gp in the game.
People do buy and sell things. That doesn't mean that there is a shop, that means there is a "marketplace". Money is a medium of exchange, and maybe the prices do fluctuate a lot... but know that there is an elderly wizard who is willing to part with his old battle staff for 100,000 gp to the party, and whether or not that makes sense, is a useful metric. Sure, the guy would rather leave it in his will for his granddaughter, but she took up with some bards and she isn't going to be using it, and he needs the money to make sure her dowry is taken care of.
Things have a value. Knowing that value is important, especially since a set standard of value would have been developed. You can say the Mona Lisa is priceless, but it was valued at $850 million for insurance purposes.
2b: Rational pricing of magical item is questionable because the utility of items is very specific and very circumstantial. Of course, a +2 weapon is worth more than a +1 weapon... but is a bag of holding worth more or less than a +1 shield?
This is tricky, but this is also why the range of prices for rarity levels make so little sense.
3: It is up to the GM yes, but player expectations is a thing that exists.
Sure, they exist, but that doesn't change the utility that the DM can get from having a more steady pricing tool to utilize. Because, one thing that we aren't acknowleding is that if you want to indicate that an item is going cheap or is worth more becuase of its history... you really can't do that currently. Aladdin's magic carpet should be more expensive than just any old magic carpet, but if you say it is worth 30,000 gp, when the price is 5,000 to 50,000... is that more or less than a normal magic carpet? How could you tell?