Because he wasn't in it!
That's right. Among other things, Fantastic Four did not feature a cameo by the man who's been called the Founding Father of Marvel comics. Here's what he said on Larry King Now when asked about the recent cinematic flop:
“It was probably because I didn’t have a cameo in it and they didn’t discuss the story with me but I haven’t seen it yet."
It seems like everyone who worked on Fantastic Four is trying to distance themselves from it.
The director Josh Trank blames the studio for their limitations, and the studio blames him for his erratic behavior on set. Rumor has it that the entire production process was disorganized and tense. Could Stan Lee's presence alone have fixed that?