I've been paying very close attention to this issue and keeping track. It is most definitely NOT the AI simply stubbornly staying on their line. It is definitely an aggressive attack on their part.
I had it happen to me in every single race during the GT500 and GT3 championship series, among others.
I saved all races and went back to view the replays.
They are definitely more aggressive.
At first I was really pissed. My thought was that no way on earth is this realistic. Any driver that purposely forced you off the track at 200mph (as has happened to me several times.) would be banned from racing for life if this was live racing.
But the longer I thought about it, the more I concluded that it's simply a new challenge to overcome.
I've found in at least one race so far that if you manage to get near the front without being spun out, the AI doesn't slow down in the last couple laps like it usually does.
Of course, getting to the front early and without being nudged off is the key!