Falling standards in American high schools
economist.com
economist.com
And yet they clearly don't, since a quarter of high schoolers were consistently failing to graduate before this trend of lowering standards started.
We've structured a large part of society to care about whether you have a diploma and not whether you've actually learned something, so it's no surprise that education systems are prioritizing the former over the latter.
Seriously, what else do you do when withholding a low-performing student's high school diploma will very likely kneecap their professional career? They don't learn a lesson when they fail out of high school, they just go get a dead-end job and stay in it forever (obviously I am generalizing a lot here, and there are plenty of exceptions). Giving them their diploma anyway at least gives a better shot of success to students who were academically failed by the education system or don't learn well in such an environment but would still be decent employees if given a chance. The rest aren't going to be materially impacted by it.