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

[Iscah]

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.

While it does make sense that they'd probably be hesitant to add another free retainer and instantly wipe off that much money they are currently earning, they are adding some amount of ancillary inventory in the form of extra glamour dresser slots. If they do give us an additional 400 slots in 6.2 as they previously indicated they would, that's going to be more than two retainers' worth of inventory space that can be cleared out.

It's actually different in many cases since crafting materials are automatically sorted into the crafting bag and not your inventory. Which is something that cannot be done with retainers and also helps take away some of that server stress they are so concerned about since ppl are generally carrying around less junk. They also have dedicated slots for various items instead of just a flat 80 items each. So, it's similar, but its really not the same. It's storage yes, but very different from a technical sense.

Youre assuming that crafting items could go into another place and somehow be treated more efficiently than standard inventory, but I don't think it's possible.

The closest thing we have to what you're asking for is the armoury chest, but it's not any more efficient at storing things, and also if you need to craft something with a gear piece as an ingredient then it has to be in your main inventory. It's like the crafting ingredient menu can't see it if it's in the armoury chest.

From what they've said in the past about the unfeasibility of a tackle box, I don't think it could store things any more efficiently than main inventory – still one slot per item, with a limit on how many things can go in, except now those slots are walled off to only hold one type of item instead of general-purpose slots with the same degree of being accessed anywhere.

Quest item inventory is way too large and just waste of "server space" or whatever they use as an excuse. Why can't that be repurposed to store tokens etc...

I've played since 2014 and never even filled the first row there

As I wrote earlier, the quest item "inventory" is an illusion and the items just appear there when the relevant quest is active. You can't actually put items in those illusory slots.