Its not that simple as you might think. 1st of all, the importance that everything except tarmac between white lines IS the part where you are allowed to drive on, for creating useful AI. I can do what ever I would like to, but then it would be hard to define edges for the AI (and you would complain AI is driving on the grass).
This is the limitation of AC, and there is nothing I can do about it, sorry.
Kerbs are almost (there maybe are some exceptions) not part of the track, and all the rest that you can find on some circuit (many kinds of run-off areas). So this is how it will be.
And you have to be aware of, that this is old school track, kerbs are high and even more worn- out in reality then here in AC mod. So it`s not recommendable to drive on most of them (in reality).