Why here is used present perfect?
frequently I have feigned sleep

Most of the confidences were unsought—frequently I have feigned sleep, preoccupation, or a hostile levity when I realized by some unmistakable sign that an intimate revelation was quivering on the horizon; for the intimate revelations of young men, or at least the terms in which they express them, are usually plagiaristic and marred by obvious suppressions.

Question

Why here is used present perfect?
frequently I have feigned sleep

Most of the confidences were unsought—frequently I have feigned sleep, preoccupation, or a hostile levity when I realized by some unmistakable sign that an intimate revelation was quivering on the horizon; for the intimate revelations of young men, or at least the terms in which they express them, are usually plagiaristic and marred by obvious suppressions.

Answer

Actually, you've got a much bigger issue — question formation: "Why is the present perfect used here?"

Answer

In the spoken language, you could certainly get away with the simple past, but the present perfect is better because of the connection to the present.
It is something that she has done at various unspecified times in the past, and that she continues to do in the present. Also note the use of the present tense in the final sentence: "… are usually plagiaristic."