everything you work on in school is not like this. you spend a lot of time studying in-memory data structures and doing tiny projects (ranging from 100 lines of code homework all the way up to 5000 lines of code midterm project) that implement in-memory data structures and algorithms to work with them.

so you get some inexperienced developer who only has their schooling to inform them and they're all stuck thinking in terms of arrays, lists, hashmaps, and trees. learning a new data model has a lot of cognitive overhead so the allure of a data persistence tool that gives you almost the same data structures you already know is very strong. "Why would I learn about relational data modeling when everything is just a hashmap anyway?" thus MongoDB as extremely popular with 23 year olds and extremely scorned by 33 year olds. 10 years of job experience drives the point home.