Some of those were unfair. X3 and Spider-man 3 were decent (though aweful compared to 2).
Batman Forever wasn't as bad as people remember it being. It had enough good moments that it can be watched every 5 or 10 years.
I think the thing that kills quality is franchise fatigue. By 3 everyone knows that the franchise is now hit or miss. People will get sick of the same characters or they might not like this one as much and avoid 4 so marketing tries to squeeze as much out of it as they can, which then adds pressure to the director.
The director is also getting fatigued. 3 is generally the last in a series. Or at least the last before a cast/director shake up. So they try to cram in as much as possible. It's no longer Raimi getting the entire Spider-history to craft. Now he's blown the origin, the best villains, and has to start digging at what's left to make a big movie, couple that with a sense that this may be the last chance to have some fun with the characters. On the flipside with batman/x-men new directors want to put their own mark on it with the sloppy seconds they're given.
Just my opinion.
