Starting from scratch doesn't mean you throw out all the good stuff. It just means that you start a new team to work on a new operating system but borrow heavily from other teams.Why would they have started from scratch?
One of iOS's biggest strengths was and still is that it is so close to macOS under the hood. That's why it got stuff like built in high performance PDF support, low latency audio, and color management years before the competition. Android wasn't really able to take advantage of a "big brother" OS with years of development behind functionality like that, and it's obvious.