You can Offline too. But to be honest i wouldnt suggest to buy the game for just playing offline .At the moment (in my opinion) YOu cant spent a lot of time with it cause after the sotrymode theres nothing you can do except some mini challenges like speed records or jumps. There arent even cool free roams cops.
The main campaign is very short, and you can complete it in less than 30 hours. There are speed traps to trigger, drift zones, speed runs and jumps to do, but aside from that there's not much else. Also, you don't get any "shipments" on levelling up if you play offline, and without those you'll really struggle keeping up with car upgrades without re-doing some of the events to get more cash. I agree with @CreativeSelfs as I don't think the game is worth the money without the online features.