who would have known & who would know ใช้ต่างกันอย่างไรคะ?

ฟังเพลงแล้วเห็นประโยคนี้ค่ะ

Who would have known how bittersweet this would taste?

เลยสงสัยว่าทำไม จึงไม่ใช้ Who would know how bittersweet this would taste?

ขอบคุณค่ะ