It's not just gpu/driver, it's library versions, compile flags, compiler versions plus all the flags (static and dynamic).

Let's say 50 options. If it costs $1 to test each combo it would cost 2^50, around $10^15 dollars. Google only made $10^9 dollars profit last year.

Now of course I'm being facetious but do not underestimate the cost of testing.

That cost assessment is bizarre. Consider how many variants are there in the entire Google software library; somehow I doubt they're spending trillions on testing.

Feel free to use other browsers that do test this.

Other browser vendors don't have Google's resources.

And still, Mozilla manages to do a better job in this regard.