Because Sub+Curse stalling is very viable with Harvest Sitrus berry, and I'm surprised you don't see any use in it. Curse isn't always easily usable, but when you can use it, it's very useful. For example, if you get a sub up, and you know a switch will be incoming (for example, anything that was trying to status you), you can fire off a curse, the opponent will switch in on the curse (and hopefully rocks and maybe spikes), and be taking hazard and curse damage, and the proceed to lose 25% each turn. So at the end of the turn they switched, you're between 50% to 75%, and they're at 75%, you can now just sub every turn, because while you're losing 25% health creating subs, you're also getting those citrus berry heals back, while they're stuck taking 25% each turn.
This leads at worst case scenario, getting really unlucky with harvest and getting 50% damage off and then having to switch and come back in a different time, or one of the two better and more likely scenarios, your opponent gets their pokemon killed by curse trying to kill you Trevenant (possible threat gone), or that you forced the switch, meaning your Trevenant will still be behind a sub, will still be at good health (thanks to harvest) and that the opponent has just racked up even more hazard damage.
Also to focus again on your point "Sitrus only fully heals the health lost by Curse if you already had Sitrus before using Curse", why would you be cursing with your Citrus berry? They come back so regularly, and if you don't have the citrus, you could be going the safer route of leach seeding.
Cursing isn't the sole strategy, but you can't always rely on leech seed/WoW, since those are blocked by either grass or fire types, where as curse isn't blocked, and will be doing a massive 1/4 after one move, instead of 1/8. If you ever have an opponent who finds out their current pokemon can't get past your leech seed + sub + harvest, they have to resort to switching while you have a sub up, and if they switch into a curse, they're either switching again (to something probably far less effective at taking you out) and taking hazard damage, or they're dying.
As the post above me said, with curse you're not just an annoying sub-seeder, you're a sub seeder that can actually do heavy amounts of damage to anything.
Hell, another scenario, set-up sweeper switches in on the turn you sub, since they can break your sub and KO you. On the turn they're in, you curse. IF they were smart, they decided to break your sub, and took 25% damage from curse, you then switch in to something to take that hit, they take another 25%. That's 50% damage, excluding hazards. If they were even smarter, they would break your sub, then switch to avoid taking another 25%, but if you have hazards up, this can mean the same amount of damage to them, and you forced a switch. If they're stupid, and try to set up, that allows you to sub stall them to death.
Having a ghost curse on something with reliable auto healing is amazingly useful.
