In any sim - everything is better when used online.
I do race online, nothing truly stops me. But there are some things that sometimes limits me from participating more:
1. Not having time even for just one hour;
2. Not being most enthusiastic about content used, give me car that needs more driving skills, hpattern gearbox, clutch, a tire that isn't made of superglue...;
3. Mostly playing rF2 so not much of it happening.... however there is one recent project called JustRace.net which seem to work very well and I already had several great races;
4. Racing with great efforts at a times is big work to do, and takes plenty of energy, which sometimes just isn't there in some days. Also in addition to that in some types of racing, like leagues, it is a must to practice properly, and with some others spending crazy amount of time practicing, it becomes kind of a job, if you feel like you must practice a lot too to remain at the right place of the competition. Even when it isn't some high profile eSports;
5. Spending too much of time modding;
6. Technical reasons - like troubles with internet;
7. Player numbers not being high enough does sometimes demotivate, although a great race can happen even if there are only two players in server...
8. If too many idiots are racing in server or generally mean and unpleasant racers, then there is no point of participating.
special cases
-ACC - lost interest, and few online races I had there was a mess and disappointing
-iRacing - in my mind irrational racing schedulles that overlaps one another, safety rating is irrational abd promotes grandma driving by the edges of a track avoiding racing, performance rating is irrational has no touch with safety and even promotes cut throat attitude, tires in most cars are terrble (hello Lotus 49), too few interestign cars that aren't even popular;
- rF2 - pickup online racing has to happen big time there !
- AC - just simply played enough, time to move on;
- Raceroom - not high enough of simulation to my understanding;
- Project Cars - I am not 14 anymore;
- Live For Speed - it probably would be interesting, but just don't even have it now;
- Automobilista 1 nd 2 - would be interesting too, but it just seem to be last sim to have space in this niche, of gamers that don't look at cars as only A to B transportation tool.