Arenas have fast allocation (because you often have no freelist and just increment the end). They also allow you to quickly reclaim old data, which is important for a database because a query might run long enough for the allocated memory to be considered old (for one example). And they don't require compaction or copying, because objects of the same lifetime are physically near each other in memory.

It's fair to draw an analogy to generational GC, but they aren't the same.

Rust is an example of a language that can do arena allocation without the dangers of C++.

EDIT: Based on my experience with postgresql, and what I've learned about rust so far, I am starting to believe that object lifetime is a fundamental concept in CS. It's possible that sophisticated GCs have, to some degree, hidden the importance.