Because it takes time. I do it from time to time when I don't feel like doing the TTs or I have finished the weeklies.
Plus I am not very good at creating entertaining custom races.
Sometimes it is the special event, which is new content. Sometimes it is the occasion to use one of the new cars on events I already completed, with some added bonus in terms of rewards.
And most of the time, the reward for completing all is enough for me.
And sometimes, it pushes me to do an event I haven't completed yet, so win-win.
I agree that rewards could be greater. (I am all for leveling the rewards all across the board to match the big 3)
I appreciate TTs, mostly for the community aspect of it (thanks to this forum, not PD), and if I am able to get into silver relatively easily most of the time, it is much harder to reach gold without spending too much time. Credit wise it is not very rewarding, but as I don't care anymore about credits, the fact that it is enjoyable and motivating is enough.
I am done with that, only missing 2 invite cars, maybe one day. And my piggy bank grows slowly but surely with TTs, tickets (when they deliver) and rewards from weeklies + normal race prizes.
Done the 4 first weeklies at lunchbreak. Was entertaining. Maybe a bit overkill with some car choices.