Is that really so hard to do?

Well, considering you can have the inventory closed, they'd have to make it force open somewhere that might not be easily visible. Along with other issues that could easily crop up. Meanwhile, it's working perfectly fine as-is. So even if it's not hard to do, it's a waste of time.

I'm not a Jagex developer (and neither are you or the other "it can't be done" players)

Yeah, so it's pretty ignorant to say something like "is it really so hard to do?" Yes. 90% of suggestions are instantly shot down (whether or not someone at Jagex actually posts) because people think "is it really so hard to do" when in reality, it is.

Also the ninja team did that big dungeoneering update already. So much for "far from their department"

Ninja team do improvements to things all the time. They find something they can do themselves (or with little outside help) and do it.

This is a job not only for the people who actually know how the interface system works, but it's also a job for the engine department because if the engine was capable of doing it, it would've been done to begin with. Even if the people who work on the engine know of a way to do it, this is, again, a waste of time for them. They have to work on much larger and much more important projects constantly. They don't have time for people who aren't happy with the shape of the inventory.

So you're asking the ninja team, a small team of people who do updates which aren't demanding of other departments to do an update that is literally not even close to their skillset. Even if they have a decent understanding of the interface system (which I'm sure they have some), I doubt they'd know where to begin with making this work, let alone know how to do anything engine-wise.

It's not as simple as just telling the code to use the already existing (which, again, isn't always existing) inventory. Even if it was, they'd still have to (like I said above) find ways of dealing with the inventory being forced open and visible, while avoiding any other bugs.

Y'know, like it being force-opened (or already being open) behind the actual bank interface. How do they deal with that? You open the bank, but you can't see your inventory because it's stuck behind the bank itself.

I'll tell you how, by leaving it the way it currently is.