The Windows 11 GUI was originally meant for a version of Windows called "Windows 10X", which was intended for low-powered devices that would compete with Chrome

The Windows 11 GUI was originally meant for a version of Windows called "Windows 10X", which was intended for low-powered devices that would compete with Chromebooks (hence the simplified, touch-centric design of the UI). Windows 10X was supposed to be a "legacy-free" OS that only ran UWP programs natively. Rather than using GDI, Win32, Explorer, etc. it used a brand new UI stack built on DirectX. This means that it required a compatibility layer to run Win32 programs, which were isolated from the OS. In mid-2021, Microsoft announced that they were cancelling Windows 10X in favor of bringing some of its features to regular Windows. This is speculation but I would guess that MS didn't want to waste all of the design work that went into Windows 10X, so they ported the new UI over to Windows 10 and used the opportunity to bump up the OS minimum requirements and drop 32-bit support. This would explain why Windows 11 feels like a bunch of junk built on top of Windows 10.