Because I accept both things on their own, and I accept that the combination of them follows naturally. Then I must (reluctantly) accept unfortunate outcome of the combination.
I wouldn't say it's reasonable that browsers could never suggest they be made default, and I don't think it's reasonable that you can't have some OS function that wants to show e.g. a help section use the OS embedded browser.
That might be acceptable if no thought went into the combination of the two things.
I accept people must work at height. I accept that people occasionally fall over by accident. We have guard rails and harnesses so that I don't have to accept people falling to their death every time they trip at height.
For example it wouldn't be ridiculous to think that if the browser is tightly coupled with the OS (to the point that it doesn't change when you set the default browser) that you can have the embedded browser opened with a no nag/no ad flag set.
Just because its 2 things you'd otherwise accept doesn't mean it isn't a dark pattern. Lots of dark patterns (like the setting up of privacy / data sharing settings on google) do similar things.
I wouldn't say it's reasonable that browsers could never suggest they be made default, and I don't think it's reasonable that you can't have some OS function that wants to show e.g. a help section use the OS embedded browser.