alt Hacker Newsit can take awhile for things to work their way from NCMEC (national center for missing & exploited children) to the appropriate local agency, then have that agency do their investigation, get a warrant (if they can), etc. and that is assuming that there is even a cooperative jurisdiction for NCMEC to send the initial report to.
but, plenty of people do end up in prison, especially if its facebook/dropbox/gmail/etc... they just dont make the headlines (especially nationally) unless its especially heinous.
I think there's also the "roll up a whole network at once" approach they prefer to take for things like this - hence nothing will happen for months/years whilst investigations happen in secret and then there'll be a day where you get 40 coordinated raids across 10 countries which lead to tens of people being arrested. Then of course there's another period of months/years where they have to work on the new stuff they've got from that raid and also preparing for trials, etc.