The problem isn't really who has the engine itself-- I'd say for most purposes, most browsers are about as good for 99% of workflows.
It's more of the forces and plays each company has around their browser strategy.
IE wasn't the best browser and didn't have the best engine, but it came bundled with Windows and was free. Firefox was a better browser by some metrics back then, in spite of having a company with less resources backing it.
Today, Chrome isn't the best and doesn't come bundled with devices, but Chrome-only sites are starting to appear (YouTube TV notoriously was Chrome-only until just this August [1]. This article lists Google Meet and Google Earth as other Chrome-only sites, at least at launch [2].
Moreover, you hear stories like these [3]:
> "I very recently worked on the Edge team, and one of the reasons we decided to end EdgeHTML was because Google kept making changes to its sites that broke other browsers, and we couldn't keep up. For example, they recently added a hidden empty div over YouTube videos that causes our hardware acceleration fast-path to bail (should now be fixed in Win10 Oct update). Prior to that, our fairly state-of-the-art video acceleration put us well ahead of Chrome on video playback time on battery, but almost the instant they broke things on YouTube, they started advertising Chrome's dominance over Edge on video-watching battery life."
So even if all browser engines were to be hard-forks of the same, each one would be developed different, and each company would play their cards differently to try to gain or maintain dominance. Google being such a big web company makes it difficult to compete against.
[1]: https://www.theverge.com/2018/4/5/17203192/youtube-tv-google...
[2]: https://www.theverge.com/2018/1/4/16805216/google-chrome-onl...
[3]: https://news.ycombinator.com/item?id=18697824