I guess historically grad school was an apprenticeship for academic research and teaching. I don't think you "need" this kind of skills (working 4+ years on one

I guess historically grad school was an apprenticeship for academic research and teaching. I don't think you "need" this kind of skills (working 4+ years on one thing with a very small team) anywhere else. Nowadays, the excess of graduate students made this _the_ convenient signalling criterion for a lot of jobs (ML at FANG, liberal arts jobs (for which studying makes real sense - think museum curator, school curriculum development)). While this has been around for these kind of jobs for ages, the glut of PhDs, etc. effectively limits access to people being able to finance their PhD and having access to the required signalling from their undergrad. In the US in general and liberal-arts worldwide this effectively means that the aforementioned positions (of a certain power) are only available to the already privileged. If you get a rare living-wage scholarship or happen to live in certain parts of the EU, this can be quite ok though if you want to increase your signalling value and do something off the beaten path.