I think it's still called just in case someone else decides they have a claim to the throne, and they can argue their points. (A moot was called upon Torygg's father's death after all. But at that time no one felt it was necessary to challenge torygg's claim to the throne.)
From UESP:
http://www.uesp.net/wiki/Lore:Moot
The Moot is a board of representatives, one from each Hold in http://www.uesp.net/wiki/Lore:Skyrim, which is convened upon the death of the High King of Skyrim to choose the successor from any qualified members of the royal family. In http://www.uesp.net/wiki/Lore:First_Era#1E_369, the Skyrim http://www.uesp.net/wiki/Lore:War_of_Succession began after the death of http://www.uesp.net/wiki/Lore:Borgas.http://www.uesp.net/wiki/Lore:Moot#cite_note-PGE1A-0 Imperial historians attribute the war to the Moot's failure to appoint "the obvious and capable" Jarl Hanse of Winterhold.http://www.uesp.net/wiki/Lore:Moot#cite_note-PGE1Sky-1 The war, which lasted over 50 years and shrunk the Skyrim empire considerably, ended with the Pact of Chieftains, which dictated that the Moot only be convened when a King died without direct heirs.http://www.uesp.net/wiki/Lore:Moot#cite_note-PGE1Sky-1 It has caused no similar problems yet.
Emphasis mine. Altho it doesn't say definitively that the Pact is still in effect, I've seen nothing to indicate that it isn't. And it's entirely possible that a Moot is called at some point after any king/queen dies, whether they are deciding on a successor or not. It would almost be expected, one would think that all the Jarls would need to swear personal fealty to the new ruler at some point. Even if that were not the case there would no doubt be a lot of "business" to discuss and everyone would want to size up the new guy/gal.
OTOH that info is apparently taken from something written in the 2nd Era, soo....