Maybe a stupid question, but if Krita can read/write .psd then why does this matter?
Even if you simply import a .psd and re-export it again as-is, it will still break things like smart objects and layer blending options.
It's not Krita's fault: .psd is not an interchangeable format. Implementing .psd support perfectly is basically implementing the whole PS itself. It's all the artists' "fault". But it doesn't change the fact that using Krita makes my days difficult.
I've once seen a .psd that won't display correctly in neither Ps or CSP, supposedly because it was exported from something else. That didn't seem like a fun situation.
It’s a 30+ year old format that went to multiple transitions. I don’t envy the poor soul that has to reverse engineer it.
This one? https://github.com/gco/xee/blob/4fa3a6d609dd72b8493e52a68f31...
https://github.com/gco/xee/commit/750196023da5457d9535b30299...
Or maybe the huge goliath that gives less than 0 shits about interoperability makes it difficult?