Is there a particular reason why we can't have ingredients inventory?

[Seraphor]

That's due to loading in your inventory along with your character and its customizations. All they need to do is add a crafting bag that isn't accessible everywhere like other games have and server load is a non-issue. They are basically using server loads and technical limitations as excuses because they would rather charge for extra storage via retainers.

Even Fallout 76 has a crafting bag. Not nearly as nice and sorted as GW2's or even ESO's bags, but I mean FO76 can do it but XIV has "technical limitations" lol give me a break

Yes that's what a retainer is, an inventory that's not accessible in high server load contexts.
We can't have more retainers, or similar styled ancillary inventories, without uprooting the subscription model and costing SE a load of income.
It's not something that's going to change, and you can call it "greedy" all you like, youre not wrong, but there's nothing they can do about it, they're not about to reduce their income are they? Would you take a pay cut to benefit your customers/service users in your job? Maybe if your customers would leave otherwise... but that's not going to happen here.