it’s been so easy so far and i set it to hard 
As far as I can remember they will automatically increase in difficulty as you progress through your career and level up.
They get harder the more successful you are - win and they get better. There is a limit to how good they got though and all you have to do is come second in one race and they will back off a bit for the next few races.
As stated they do get slightly harder the farther you get in the World Tour. Once you complete the tour, then go into the event list and you can race at whatever difficulty you have set in those races.