Well, the same could be said about Gears of war or halo really...
So let me ask you, are the stories in the campaign worth it?
Are there diverse weapons to try without having to dive into MP?
Are the mechanics (shooting, fluidity and whatnot) on par with other similar games?
The offline game guarantees that you will start out with your characters default weapon, and you can equip them with any other claim you earned from online mode.
The story line is fun to see, a lot of good cut scenes both at the beginning, the middle and the end of each mission. It is fun to play through, but knowing that there are other items, upgrades, stat perks you could be getting makes the offline experience non-appealing for more than the one time gold run. You have to realize Halo had no benefit for playing online, you got nothing really for it other than achievements, not sure about Gears. With those games, as well, you could play Co-op mode online with no PvP if you didn't want to, you had the CHOICE of PvE and PvP. You weren't forced. And so the experience for a campaign player was really the same ofline or offline and you had LESS incentive to go online other than the thrill of it.
Also with raiders, this is a Hero Team Shooter/beat em up, not an FPS. You sound like you are used to games where you pick up enemy weapons (I wish I could get the mooks assault rifle, good at all ranges). You only have your one characters weapon, you don't pick up new weapons from enemies. You only get different weapons by earning them through loot rolls at the end of missions IF their "Blue Print" drops, and, sadly, this is only online.
The mechanics offline are even better offline. It's a very fluid game, but again, is not a straight FPS, it's got a lot of special ability mechanics and melee combat is a BIG thing in this game. It doesn't play like Overwatch or Unreal where you are basically a floating turret that moves with no weight or gravity affecting it. Your character is very "physical" and can't turn unrealistically fast like other shooters, nor can you run at full speed and then change direction on a dime, you move more realistically. But this is a good thing considering the nature of the game.
I happen to like the story line. There is an overarching story line and then three current campaigns, each with 4 missions, that have their own unique plot arch.