Jack's return was staged as a movie, but could easily be redone as eps that are extended to the arc.
Arnold could've had a miniseries Jungle arc yea, but Zim nah, it'd be mismatched with a very strong story instead of a hunt kinda thing. Same for Rocko but instead just because the whole plot does NOT sync into episodes, just a self contained special.
Nick was only doing this for nostalgic events to extend on the 90s Are All That/The Splat gimmick, the fact they picked some shows with pre-existing arcs that needed to be finished wasn't directly their main goal, it just worked with the other shows getting revivals.
Jack is a building thing, it works fucking GREAT. Besides if Nick brought back like 4 cartoons for short seasons it'd also look bad and cheaty in comparison. Very try hard.