Your question is why do we use the "present continuous" (also called "present progressive") and not just the present ? The present continuous is used when emphasizing a temporary event or the temporary duration of an action. Example: "The film is starting now." or " I'm getting nervous." If you say, "He is sarcastic." or "He is a sarcastic person," the implication is that this is a permanent trait or attribute of this person. If you say, "He IS (just) BEING sarcastic," it implies that only at this moment in time is he acting sarcastically. He is not always like this.
"Is she always like this?" "No, she's just being silly." or "Yeah, she's a silly girl."