And there is of course the question of safety; should you release an arena too early, you may have introduced a bug. Worse: it might not crash immediately.
There is actually some work for doing arena management automatically, called region inference: http://www.mlton.org/Regions
But the way I see it, it's just a way to make memory management even more efficient; it's not about precise release of resources, and indeed not all programs can be expressed so that releases can happen only in batches of an arena (assuming those arenas themselves aren't dynamically managed, which certainly is a valid strategy as well, but manual).
A memory safe programmming language will detect any such bugs and reject the program. This is not hard, it's a clean application of existing lifetime checks.
Note that you don't need GC to reap these benefits, if desired. You can allocate an arena and do secondary allocations inside it, then deallocate everything in a single operation. Arena deallocation is not timely or precise, but it does happen deterministically.