What reasons would they be then? I've already explained that you are more likely to finish in a higher relative position from a grid start than you are one of these unrealistic rolling starts where the cars are far apart and in single file. OK, so the cars cross the line at 100mph+ or whatever meaning you get a bit more time at race speeds...big deal! It only takes a few seconds to get up to that speed anyway from a standing start. So, in what capacity is a standing start in a 2 minute demo not suitable?
Not only that, with only 8 cars on track in a lot of these demos, I can't name many tracks we have seen where grid starts would be physically limited and impractical.
Honestly, I'm hyper excited for this game, but I'm not going to defend every single aspect of it just because of that. I sincerely hope that they do put grid/rolling options in for us, and that in the final game, rolling starts are more realistic and not some precession of cars in single file all patiently waiting to cross the start/finish line before accelerating.