Campaign is short. The older Call of Duty games were longer though. Most of the fun is online though. Where you get the unlocks and everything.
I would take it further and say that seems to be the trend for most games. Imo most solo games are short. I stopped buying games without an online feature because they're always so short.
Also, Flip is right, the real fun is online. And to me the actual real fun started when I stopped trying to be a "soldier", crouching and sneaking past corners etc. It's fun to be a ninja sniper and sneak around, but it's a lot more fun to run around killing people with the throwing knife or the combat knife. Or akimbo 1887 shotguns.
Make a character with the marathon and lightweight perks, then find a suiting one for the third (i like ninja or jamming for the purpose). Equip a .44 with the combat knife attatchment and run around knifing people. You'll run super fast. Learn to run in zig-zag when running towards people. It's so fucking hilarious when you get the hang of it, to get "most kills" and "killed entire enemy team" with just the knife. When you get 3 or more kills in a row it's satisfying to think about the nerd rage the guy watching the kill cam is going through
A last little trick, set one of your killstreaks to be the airdrop, then equip the red smoke grenade when you get it. When you have that equipped you can still stab, but you run even faster. Not sure if it's a bug or what but no one can ever catch you. It is hilarious.