In almost 2 years, since the start of Early Access, I have been playing ACC and never once have I played online.
As a skillful racer, my reasons for not racing online are the following:
1) Not convenient
Online cannot compare to offline racing when it comes to convenience. When living a busy schedule, if I only have 20 minutes of free time, it's not practical to go online and start to search in hope for 30 other racers who happen to be available to start a race immediately and finish at the exact time that I need to finish. Furthermore, if I need to leave the house mid race during offline, I can pause, save or quit anytime without worrying about gaining the reputation of an online quitter. I can eat dinner whilst playing offline, pause the game multiple times. Send an email mid race or make a phone call. Go to the toilet, come back and resume. You don't have anywhere near that level of convenience and flexibility when playing online.
2) Horrible experience
Never again will I play online in a racing game. From previous limited experience and judging what I see on Youtube videos, online competitive racing is usually a substandard experience. Everything seems to go downhill. Graphics. Framerates. Cars teleporting or micro-stuttering around the track. Excessive amounts of divebombing. Human opponents dangerously swerving their cars left and right and back again, trying to prevent passing, which wouldn't be allowed on a real race track. I must mention divebombing again as its frequency in online sim racing is on a high level not seen in real life racing or in ACC offline against AI. When ACC first released, its AI was almost as horrible as online racing with its divebombing, but now much more enjoyable.