Aside from the negligible minutiae of how things are written, there is literally no practical reason to treat origin characters any better than the custom made ones.
The effort that went into the origin stories could have been better used making the tags feel more relevant and/or having more of them.
I'd gladly have a 'merchant' aspiration instead of Ifan as a thing at all.
Totally this.
And honestly Stabbey, ask around, how many people want to have a preset character instead of a custom made one? Larian is bragging about their ability to create pen&paper like rpg games, an effort could have been made for the tag system. This is just a question of how much energy you want to put into this, there's no real impossibility here...
You're right, it's a matter of energy. Six fully-fleshed out origins which dramatically alter many NPC reactions is already extremely ambitious. There's nothing impossible about adding a dozen tags to create interesting custom characters, but it's a ton of work, more than making a single origin to make it meaningful. Maybe they oversold the idea of creating your own background through tags (though I can't recall how big of a deal they made of that), but perhaps they'll be adding more tags.
If I were to make a wild guess, making custom characters meaningfully customizable in their background would probably be about the same work as two to three origins. I would say an origin is probably worth 3-6 tags in terms of uniqueness, depth, and breadth of effect, so you'd probably need at least 10 tags you could choose from to make your character feel unique, some of them not part of any given origin, since origins also have tags. I'm not sure how many tags are in the game already or exactly how large of an effect they have besides dialog options, but you'd definitely need quite a few more to to really do custom characters justice.
Which, to be clear, would be ideal. I'd like deeply customizable character backgrounds as much as anyone. And maybe that's Larian's plan, but I wouldn't necessarily count on it. Most people will find six interesting origins with lots of choice in their development to be quite generous as is.
One easy way to make customizable characters special would be to let them choose from some special talents at the start of the game that have large stat effects, mostly with certain drawbacks. Say, something like the "Merchant" background would grant you 3 Bartering and 1 charisma but reduced constitution, or that sort of thing, perhaps even granting some unique effect like merchants have more things to sell. Ideally you'd gain some special dialogs with other merchants, where it'd also be a tag, but that's moving into the less easy way to make customization. Mere gameplay effects nothing equivalent to an origin's story relevance, but it'd offer some sort of flavor to custom characters.
