I noted this in my other response but since when is Google the king of the web and in what way do you mean that?

Mozilla seems to do the most work trying to move the web as a platform forward.

Flutter is a system that would be impossible to create or fund unless you were at google scale, because it's massively wasteful as an idea -- it's literally a complete rendering engine and widget system built from relative scratch. It offers more flexibility than adhering to any other established UI standard, so I'm not surprised it made sense for them -- surely they can move faster with Flutter than they can by trying to support all of HTML.

There is a cost to using established standards and being held accountable to outside forces, Google just chose not to pay that tax here, which is well within their rights.

I think both Google and Mozilla are moving the web forward. But I'm using "king" as a metaphor for power+money.

No one derives more power and money from the web than Google, and therefore they have a vested interest in keeping the web at competitive levels of performance compared to native platforms.

So if neither Google nor Mozilla can make web rendering as fast as Flutter, then I suspect that there is an inherent technical reason for that and not just lack of want.