What is the reason for the change in the overall graphic design of a new version of an operating system?

What is the reason for the change in the overall graphic design of a new version of an operating system? 
Why not just keep it the same, so that users don’t feel like they have to relearn the whole entire system?
My Answer
Long time ago computer scientists worked on Networks powered by Dark Terminals.
The idea was to share information for furthering developments and enhancements.
Working on Terminals powering Networks or Servers was fast when band width was narrow.
With the increase in technology band width and graphic capabilities improved.
My first assembled computer to my design had 4 MiB Graphic capabilities and 128 RAM.
Only Linux satisfied my working routine with an ugly desktop.
Mind you computers work on RAM and graphic card was a solution for freeing RAM for critical work.
I hated black terminal an an early adopter of graphic front starting with Gnome.
I became addicted to KDE and currently love KDE Plasma in Debian.
Graphic front improves productive work and background seamless (millions of lines) coding user does not need to know in detail.
Whole idea of the graphic front is for User Friendliness.
After all one does not wear the same cloths everyday not even Linux Developers.
I love to dress flashy cloths and it applies to each distribution and its flashing clothing attire.
One picks the one that pleases ones aesthetics or EYE.
One size does not fit all (unfortunately that is the philosophy of Windows and Apple).
Well one can use one toothpaste to brush ones teeth.
The idea is to have beautiful set of teeth or a denture when one smiles.
Windows is like an ugly denture outside the oral cavity (mouth).