I contracted the iRacing sickness this year. The online is indeed unmatched but I’d argue the single player racing is also best in class.
The iRacing AI is actually fun to race against and nothing else comes close to the level of customization per-racer. You can build whole custom rosters with individual behaviors. If you’re so inclined you can even share all this stuff through communities like Trading Paints and Race Department.
iRacing is a hole with no bottom. Both time-wise and monetarily. Even to do AI-only you’re still paying the subscription and one-time content prices. It really is the best though.