In Iolanthe why do the fairies sing, ''To say she is his mother is an utter bit of folly'' when they know Iolanthe IS Strephon's mother?

Probably because -- "The fairies laugh that Iolanthe appears too young to have a grown son, as one of the advantages of a fairy's immortality is that they never grow old."

EDIT: Because it's a comic opera?

"It doesn't make sense to me" --- It's a comic opera. It's not supposed to make sense.

''To say she is his mother is an utter bit of folly'' --- It's only a bit light-hearted flattery with a hint of sarcasm and/or irony: one of W.S. Gilbert's (Sullivan's librettist) trademarks, which was never intended to make sense or be taken seriously.

Added: Hopefully, '13Across' will see this question and give you a better answer. I'm doing my best to explain it, but English isn't my native language and I can't always make myself clearly understood.

Yes, but the fairies know this fact, so why don't the fairies say. ''To say she's NOT his mother is an utter bit of folly''?