A lot of people requested it, but maybe Larian didn't want to introduce new gameplay mechanics at the last minute, because the game was designed to let you skip all the RTS battles, and even if you play them, you can skip playing as the Dragon too.
In their Divinity 2 behind the scenes video, they mentioned that initially, they wanted an RTS at the end to be part of the final battle, but that meant they needed to teach players the mechanics and such.
