Because just like the most "open source app reading/writing proprietary format" scenarios, it doesn't do it perfectly.

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.