What is the peculiarity of registers and why are there so few of them?

Question

Iron

Tell me, what is the peculiarity of the registers?
And why are they only on the fastest processor cache?
Shouldn't they be in every data branch in RAM before the data exits?
And why all this, if each input-output element receives a contact from the frequency of the processor?

Answer 1

Tell me, what is the peculiarity of the registers?

They are very fast. And very expensive. Therefore, they are few.
This and everything else is an incoherent stream of thoughts.