I thought it was because Apple still put so many roadblocks in the way of browser developers that nobody was able to pass them.
But the bigger the market they can reach, the bigger the reward, and so at some point it may justify investing resources to work around those roadblocks and accept the drawbacks.
TBH I'm fine with that. Applications, browsers or not, should use the operating system's components and APIs for things for a unified experience across all apps and interactions. On the desktop side of things, I hate when an application breaks convention for the OS it's deployed on. If I'm using macOS, for example, I want every app on my mac to look and behave like any other, consistent with the rest of the OS.