What's the point of having to upgrade most items between every two levels? Is it actually fun for someone?

I do like building the team, but it starts to feel pointles when I know most items I find or buy at high pricess will be obsolete after few levels. It's also quite immersion breaking to know that the same merchant I'm trading with will have superior items for sale after I get some more experience.

I'm not sure if I get my point through here, but here's an example:

In Baldur's Gate 2 you can find Celestial Fury rather early on if you know what to do. It's a super awesome katana that can carry you through the whole game. That's something unique and memorable.

In Original Sin 2, no matter what legendary you find, it will be practically useless and forgettable after few hours of gameplay at best.

Another game that has the same leveled items nonsense is Witcher 3. It's really dumb that Gerald may find some 'Super Meteorite Silver Sword of Vampire Slaying', but refuses to use it, because it's at some artificially high level of experience.

It just feel too gamey and artificial for me, what's the point of this system?

Overall both DOS are great but itemsystem&Inventory handling are two drawbacks for me.
I think the point of the system is the diabolotouch, many people like it to find great stuff all the time..and even greater stuff and so on.
I really liked the Itemssystem for BG but many gamers i know found it boring.