Ooooooh!
For years I always asked myself, "Why couldn't Sonic Team simply recycle Sonic Generations into Sonic Forces? It's gameplay, it's physics, it's assets for Green Hill and Chemical Plant." If the answer was simply that Hedgehog Engine 2 could not import all that stuff then that would be a logical answer.
But then again couldn't they import the raw assets they already had for Generations into Forces? I dunno. I just always thought the Forces versions of Green Hill and Chemical Plant just didn't look as good as Generations versions... though Forces Chemical Plant reminds me a little of Arkham Origins.
Time constraint probably.
Since both Forces and Hedgehog Engine 2 were developed at the same time, Forces was developed with unfinished engine which still limited & have many restrictions which make making new elements in the game was faster than working around with the engine to accept "not-yet" compatible assets..
If they wait until the engine finished it would be too late to deliver the game at the promised release date.
I'm sure they'll rather choose the easy way if they could but they couldn't.
Well, the engine was finished (and has been used for new Sakura Wars, Mario & Sonic at the Olympic Games Tokyo 2020, Olympic Games Tokyo 2020, Puyo Puyo Tetris 2, and Sonic Frontiers) so they now could focus on Frontiers alone.