Equipment is produced in towns by workshops.
Artisan workshops, which are found in every town, produce most manufactured items in the game, including all weapons and armor. Smithy workshops will also produce all melee weapons as well as armor that is medium quality and up, and Wood workshops will produce ranged weapons and shields.
The rate at which workshops produce certain equipment depends on the tier of the item. Both Artisans and Smithies produce tier 5 and 6 weapons once every 10 days. These workshops also consume certain inputs from the town's market in order to produce an output item, and if the input items are missing, then the output gets skipped and production restarts at day 1 of 10. To create tier 5 and 6 weapons, Smithies require 1 iron per weapon and Artisans require 1 iron, 1 leather, and 1 hardwood per weapon. Last I checked, it was not uncommon for these workshops to have a general shortage of the input items required to make high tier equipment, so their production can be sporadic. This is partly because when workshops create their products, they produce lower tier equipment first, which requires roughly the same amount of inputs, so by the time the high tier item production happens, the necessary inputs are often already consumed. For instance, every 10th day, a Smithy will consume 1 iron a piece to make tier 1-4 melee weapons, and if a town doesn't have more than 4 iron on that day, then tier 5 and 6 weapon production unfortunately gets skipped.
Selling more input items to towns might help ensure workshops don't run out, but these items will eventually get consumed by the townsfolk themselves, and it's impossible to know what day a workshop will produce the tier of item you want, so it can be difficult to coordinate.
When a workshop successfully produces equipment, the game will randomly select an item out of the pool of items in that tier, but it's weighted by item value, meaning higher value items are selected less often than lower value items. Without double checking, I think tier 5 and 6 weapons are lumped together in the same pool for the purposes of workshop production. Since the Long Glaive is tier 6 and is one of the most expensive melee weapons in the game, it will get selected comparatively less often than some of the lower priced tier 5 or 6 weapons, hence added rarity.
Also, most equipment has an assigned culture and will only be produced in towns that belong to that culture, so you'll only find the Long Glaive in Khuzait towns (which you've already correctly figured out), further decreasing the number of opportunities for one to spawn.
The workshop balance gets tuned fairly often, so some of these items might be too rare at the moment, but will hopefully become more common with later patches.
TL;DR, the Long Glaive can only be produced in Khuzait towns by workshops once every 10 days, and is rarer than most other melee weapons because of its high value. To increase your odds of finding one, check towns with Smithies.