But at the least the decision is the same across the OS. If you spend several hours using the system you will know the "right" way. But in the web you're on nowhere's land.
There are lots of other design sins this doesn’t excuse (low contrast, for instance), but expecting conventions to be as consistent as on a desktop OS feels naive.
There are some things where the browser will apply platform conventions, but nothing resembling the consistency you'd expect from (well-designed) native apps.
[1] https://developer.mozilla.org/en-US/docs/Web/CSS/system-colo...
Nothing stops Web developers from flipping the buttons if they detect a Mac, but that would make things more confusing if someone is using the same Web app on multiple platforms, or if the docs have screenshots from different platforms (to each other or to the user).