Because the required level is shit. And it has been downhill since the implementation of modern math and the rejection of math in the modern era or elitist path at school. We see it everywhere and especially in France or Switzerland. 10 years old native kids who can't read fluently. 15 years old who would write 1/2+1/3=2/5. They will keep this level of skills through the end of high school and master degree thanks to…
On the other hand most people in the 50/60s would also fail the brevet (or baccalaureat for that matter). I can't speak for the math level but I do know that school in the 60s required you to learn every single prefecture and chef-lieu by heart in geography classes, and the name of every single bone and organ in biology classes. Sometimes modern reforms aren't all that bad.
One fault is the drop in teaching hours : https://www.reseau-canope.fr/musee/collections/cache/a65f40c... this is a schedule from 1952.
We are lying to kids and parents : https://twitter.com/loysbonod/status/1356128734508679168
Or Pierre Colmez writting : https://webusers.imj-prg.fr/~pierre.colmez/lettre.pdf and https://images.math.cnrs.fr/R-eduire-les-in-egalit-es.html?l...