Why are goblins bad for war?

In Clan Wars, stars are everything. That means you need to try and wipe their base, rather than just raid it for resources. Remember that stars are only awarded for destruction of the town hall, and for 50% and 100% base destruction.

Goblins are good for raiding resources. They hit collectors and storages for extra damage, but they are relatively weak otherwise. Mortars make short work of them, and they can't really hit walls well enough to break through.

Most other troops either hit defenses or don't have a particular target they prefer. This means that your goblins are likely to split off from the pack and attract the attention of other defenses.

Thus, goblins are bad for war because they're not aligned with the goals of war raids, they're weak, and they're prone to running off by themselves and getting splatted to death by mortars.

Goblins aren't the best but they're certainly not bad.

Many players dislike them cause you won't get many trophies using them and in Clan Wars the major part is getting the stars! But in some cases, they are definitely neat and strategical in small amounts for accumulating resources (which isn't that important).

Goblins are also the fastest troop in the game, and are also great at destroying resource vessels. They deal double damage to: (Gold Mines, Elixir Collectors and storages for each)

But despite being good at taking resources, they lack at pretty much everything else and the main goal of getting stars, and that's why they're disliked by people.

Using goblins isn't about collecting resources, it's about using the goblin's attack strategy to your advantage. There are many special needs situations that goblins can fill, but they require advanced strategic thought and execution. It's not for everyone. If you're just going to spam goblins in a War, don't bother. If you want to use them to trigger big bombs and spring traps, practice a lot. If you want to use a couple for diversionary tactics, practice a lot.