It isn't because there are too many Spider-Man movies though. It is because there are too many bad Spider-Man movies.
Harry Potter had eight in that span and never experienced fatigue. Iron Man is about to have his fifth in seven years next year and people still want to see it.
Also look at Batman and X-Men. In both cases they started off well. Then they started making bad movies (Shumacher Bat films, Last Stand & Origins) and people left. Then they started making good movies again and people came back. This is not coincidence.
The 'fatigue' is entirely quality related.
Fatigue has never been entirely quality related. Whether its a sports person who becomes burnt out playing to much of the game they love. Or a desert you're sick of even if it might be your favorite.
You can have to much of a good thing. When so many films are put out in a short space of time they lose there impact. There were 8 years between Nolan series and the one before.
In North America it may make sense but ASM2 seems to have had a decent reception in most places. I'm interested to know the reasons for the discrepancy tbh.

