You need to understand that SketchUp has a long history. The "inventors" were a couple of college students fulfilling a coursework project. It was very simple in the beginning.
They saw a need and started a company, and began improving the application little by little. But they have always had a philosophy of simplicity. In those early days, they didn't really envision professional engineers using it the way they do today. So, pls give them a bit of a break, when it comes to the "I saw it needed such and sucha feature immediately."
(I noticed many missing features myself coming from using CAD, Database and Office software for 30+ years, but those guys didn't have this when they started out, and were only trying to create a simple easy modeling applet.)
Then the company changed hands twice, and each new owner has had different ideas on what the goal of the application is. Trimble is an engineering services company so features are getting better, but it has taken some time to clean things up "under the hood" that stagnated whilst Google owned SketchUp.
Take some time to read the both Application Release Notes and the API Release Notes from v8M4 onward, and you'll see a drastic change in the amount of work and the direction of changes once Trimble acquired SketchUp.
And... You should read this thread, ... it explains well their core philosophy:
With regard to nesting of layers and scenes, ... the issue is compounded by import and export concerns. The hierarchy of the model objects need to translate to other major formats (DXF, DWG, DAE etc.) or be "flattened" according to some rules that everyone can agree on. (I'd perhaps like any ">" character in a single level format layer or scene name, to import into a nesting allowed format as a nested layer or scene, whichever it is. And visa versa. But the external formats would need to not choke on such characters in layer or scene names.)
The same holds true for any backsaving to older "flat" SKP versions, or upsaving or component loading into newer nestable versions.
Add to this graphics rendering issues. It's just not simple, and will require extensive testing when implemented.