"If gnomes are real, then are fairies real?"
A worldly gnome:
"Well, of course. All living creatures are real, silly!"
"Then how come I don't see them?
"The fairies or the gnomes?"
"Well, you don't see fairies because there aren't many around here anymore. They mostly moved to Hollywood (and various Disney locations throughout the world). You don't see gnomes because they sleep at night, are careful to blend in and - probably the biggest reason - humans just don't stay still long enough or look carefully enough to see them.
"Oh. I thought it was because they could make themselves invisible."
"Well, that too."
For more details on why the fairies all moved to Hollywood, read: