The point of SAP is that the module for your type of business is being implemented at the market leader in your area, because they are the only ones who can afford it. SAP will come to the market leader and put their processes into software.
Then everybody else adapts to SAP not because adapting SAP is impossible but because SAP implements the processes that made the market leader successful (at least that's the idea).
So it is very intentional that you adapt to SAP and not the other way around. You do it to emulate the market leader.
And the user friendliness is also on purpose. If you go visit a company using SAP, you will find that you have a certain job in the company and it needs two or three things from SAP. Jobs are very specialized, so SAP is, too. Nobody is supposed to just wing it with SAP. You are supposed to learn how to do your transaction with SAP, and that transaction is then meant to be super efficient.
If you actually go visit a somebody from an enterprise environment interact with SAP, you will be astounded how quickly they can do what they are supposed to do. It's almost as if the software was optimized for frictionless efficiency.
Also note that user friendliness is a subjective and mysterious area. If you go look at an airplane cockpit, where actual research was done into how to optimize user interfaces so that the pilot will not be confused under stress, you will see that there is a separate button for everything. There is no "shift key", no different modes. If you need to use it, there is a button for it. To someone who has not learned how to use that UI, it appears alien and unfriendly. But the the trained pilot it is the pinnacle of perfection.