None of these bands would resurrect, unless there was still a demand for them. I guess a lot of these guys have thought, oh well, my heyday is gone, so I might aswell do some other musical project (like Mike Patton with a gazillion different bands...)then it might come as a slight surprise that people are still interested in FNM/SG/STP, and that alone can give them the confidence to go back to where they left. And to my understanding, the break-ups of bands like Soundgarden etc. weren't that messy, but everyone just wanted to do something else for a while, but then someone makes a good offer, and it's "why not".
They've seen their club days long ago, why go back to that? But (as in stp's case) "wanna make a million dead presidents?" sure, why not! it's a win win situation, and even though every show might not be the most fun to do, atleast I've got the picture that stp still enjoy majorly what they do and would they be weeks away from their families if they got paid a six pack of beer?