So? Just because some optional classes don't use optional mechanics doesn't mean that none can.
But it doesn't mean they have to either.
Because the system as it stands doesn't support it. Similarly, if and when they implement optional Psionic classes those classes will necessarily rely on the optional Psionic subsystems.
The "psionic subsystem" is entirely self-confined to the class. It changes the world, but doesn't really change the game. It does not affect balance, nor does it affect the other classes or characters. And you can insert a psionic character into an established module easily and effortlessly.
A character that can make permanent magic items and craft items change the game. It break organized play, changes the balance, and has a dramatic impact on storyline modules. It requires the DM to adjust the game and not just the world.
That is bad design, plain and simple. As poor design of an option as something with blatant power creep.
And it's telling DMs
how to play. The artificer as a concept doesn't require permanent magic items, potentially beyond potions or maybe scrolls. That's a fun concept that can fit many worlds without the magitech of Eberron, such as Ravenloft alchemists or Dragonlance tinkerer gnomes, both of which have rare magic items.
It's a hefty change for the sole benefit of matching the design of 3e.
Heck "artifice" first appeared in 2nd Edition, in
Spells & Magic. Why not use that class as the basis? It's just as valid a source of inspiration. So no magic item creation and spells of the Enchantment and Necromancy school are banned.
They don't need to use the same system. They just need a system to do the same job.
Eberron was always a world of common convenience and low-powered magic. Commonplace mundane items like lamps and magic brooms. Higher powered magical items like +1 swords aren't part of that. They were there for math reasons.
There's no problem with allowing the artificer to create small cosmetic magical items and effects, like the rock gnome. Making common magical items that have no heavy game effect. Ditto making potions, wands, scrolls, and similar consumables.
They could have an "infusion" ability that lets them impart magical abilities into an item for a time. Spend a short rest to give a sword bonus fire damage or the like. Have a list of known infusions that allow some customization. Something that's different from the crafting rules but doesn't allow the artificer to just set-up a magic sword store or give the party stat boosting items or turn the DMG into a shopping list. Or force the artificer to have endless days of downtime to be effective (by equating a party's power level to the pace of the campaign and amount of disposable gold).
But that's very, very different than "crafting". It has similar weight at the table, as the artificer chooses an enchantment to apply to an item from a list, but it can be more carefully balanced.
And just because parts of the system are "simple and fast" doesn't mean that all elements need to be including those optional elements. During the 5e design the teams talked at significant length about supporting different playstyles including those who prefer greater complexity.
And when making something as big as a class, it should accommodate different playstyles and types of campaign. It should have as broad appeal as possible to justify the investment of time.