Eh, I'd need to see some solid statistics before making a generalization like one genre being longer than the other... Just relying on anecdotal evidence and examples isn't going to do much.
Assuming it's true, I think the main reason is that in WRPGs, the player has more choice in whether to do non-main story stuff, while in JRPGs almost everything is forced into the main narrative, even if it doesn't do much to advance the narrative itself. A WRPG says "here's the door to the next chapter, but you can stay a while in this area and bring this kid some flowers, if you feel you want some more XP". A JRPG says "here's the door to the next chapter, but wait! A child is crying that it wants some flowers, you can't progress without that! You want to go through and deprive a child of joy?"
And this is why...
endtherapture said:
But a standard JRPG like FF or Valkaryia Chronicles will easily stretch past the 50 hour mark, but most WRPGs are around the 20-30 hour mark but you might get the odd outlier (50 for Dragon Age Origins, about 90 for Skyrim)
... you need to specify how you calculate this stuff. You say
Skyrim is 90 hours long, but
Spoiler Warning did a playthrough in about 20 hours. You say
Dragon Age: Origins is 50 hours, but my complete playthrough took me 67. On the other hand, you make a broad statement about "standard JRPGs", but I remember
Star Ocean: The Last Hope being around 30 hours at most.
My point is, without some kind of "standard measure" we don't even know if the main assumption is true. Wasn't there a website where people logged their completion times for various titles? Averages from that could be useful.
On a slight off topic, why does everyone keep calling
Valkyria Chronicles a JRPG? It's a squad-based tactical game, like
XCOM or
Jagged Alliance. Is it because of the anime-ish style?