Weapon levels are a feature of many RPG's including previous Divinity games. They are part of the game.

Some RPG's calculate most damage with character attributes and magical bonuses.

Divinity is a series that determines most weapon damage from the weapon as long as the wielder has the attributes to wield it.

The advantages for the dev's are in the level scaling and balances of encounters within the RPG using this method. Divinity games use random loot and the leveling of the weapons provides greater variety within that. For the player it is in the feeling of equipping that new weapon (or the one you've been spending attribute points to wield).

It's certainly not meant to be a realistic simulationist RPG.
In terms of comparison to pen and paper RPG's it's certainly not RuneQuest.

I'll do a quick comparison of 2 pen and paper rule systems, traditional d&d and 13th Age. 13th Age uses damage by character level and class but in terms of the difference in damage by level scaling it's broadly similar to here.
A traditional d&d type game would have a warrior increasing damage mostly from enchantment and training and increasing the number of attacks, so a 8th level warrior might attack twice for 1d8 plus static modifiers. 13th Age would have a warrior make one attack and roll a number of damage dice by level (so at level 4 it's 4d8+ with a sword or damage equal to level on a miss)*. The RPG systems then balance opponents by level against these scales. 13th Age designers changed that from traditional d&d because melee classes kept falling badly behind magic at higher levels and combats got delayed by multiple attacks and calculating multiple modifiers for no real benefit, especially when it was never a simulationist game in the 1st place. So, in terms of damage scaling only, this is closer to 13th Age than d&d.

If they implemented a flat weapon damage with damage increases coming from static modifiers here, I think it would make for a lot of work for no real benefit.

It's not wrong, just different. Please also remember they are still balancing the game.

*I picked 8th and 4th because 13th Age only has 10 character levels vs 20 for traditional D&D.