They are just a very low drop rate.
There was a recent patch that did mention tweaks to their spawn because they may be aware the drop rate was shared with so many other items.
https://forum-en.gw2archive.eu/forum/info/news/Update-Notes-October-22-2012
“Increased the chance to get vanilla beans from herb nodes in the Tarnished Coast and Steamspur Mountains.
Increased the respawn timers for omnomberries, black crocus, ghost pepper, lotus, and Orrian truffles to 5 hours.
Reduced amount of cinnamon dropped from nodes from 3,5 to 1,2. Cinnamon now only comes from green wood nodes.
Reduced amount of walnuts dropped from nodes from 3,5 to 1,2. Walnuts now only come from soft wood nodes.
Removed parsley and thyme from herb nodes in the Tarnished Coast and Steamspur Mountains.”
Coupled with this info I would say that Tarnished Coast is a good place to try because there were two tweaks in that area (multiple zones).
