Many ignorant players just pick pearls and leave rest, so node dont respawn. Same with chests and all other nodes, including thieve troves and psijic portals.
Oh so true, and so VERY annoying.
A minor nitpick. Partial nodes do respawn eventually. I'm not 100% sure if they only respawn on the interval, if they chew up part of the zone's population limit for that node type, or if they have a timer and despawn after a fixed time has passed. However, the myth of people picking all the good stuff out of nodes is, just that, "a myth." In many cases, we're talking about drops that are extremely rare to begin with, so it's not that someone went through and grabbed all of them, it's that the items weren't there to begin with. (For example: Nirncrux, Aetherial Dust... oh god, the conspiracy theories I've heard about people hoarding all the Aetherial Dust are equal parts hilarious and sad.)