Because if you are deworming at inappropriate intervals, with ineffective drugs, or under dosing then you are actually not helping yourself. With the fact that 2 of the 3 commonly used dewormers are no longer very effective and the 3rd is starting to show resistance issues as well, we are steadily running out of drugs that are going to work. There is only 1broad spectrum drug on the market so far that doesn't have any resistance issues--moxidectin--but it's in the same chemical family as ivermectin and it's only a matter of time before resistance to it developes. There are no new deworming drugs scheduled to come out on the market. So what are you going to do when all of these drugs are no longer effective?
The more often you use these products, the faster the resistance will develope. So we need to focus on getting our pasture contamination minimized so that our horses aren't picking up as many parasites rather than just counting on regular dewormings to keep the parasite loads low.
We worm our horses every 3 months with different wormers. Being honest, it's really hard to find a stable around here where the horses are wormed more often than that. Most of them only do it twice a year....
Doccy lived at our place for about eight months or so and in those eight months I think we wormed him ONCE. And I rode him outside of our property pretty much everyday, for about two to three hours, all over the place, and he was always fine. Never ever had worms.
But I was told that the main cause of worms is from manure (apparently they breed in manure). We clean our yard every day, so that might be why we don't have to worm so much.
But yeah, reading everyone else's comments, three months sounds alright. I'd have to honestly say I usually wouldnt worm them more often than every three months.
Maybe ask your vet too, just to get a more educated opinion ;)