I'm still in high school so the scale of everything would probably not be an issue although I see your point of maintainability and showing the relations between classes graphically is probably also helpful when you're still wrapping your head around OOP concepts
But it won't last forever, and what it does well - conveying often unintuitive OOP concepts - it does do well.
Your course conveners job its to teach the fundamentals, tooling is for you to shake out as you gain experience and scale up, and from their perspective - imagine trying to wrangle random IntelliJ/Eclipse issues with a cohort of 120+ students, for many whom this might be their first introduction to programming!
Scaling things back probably genuinely allows them to provide a better learning on ramp.