You can save memory by writing a file a byte at a time rather than allocating a buffer. The reason you don't is that it's more efficient to batch that work. Well, the same is true of GC. If you have a million dead objects and a thousand live ones, doing bookkeeping for each dead object is far more expensive (in cycles and bus bandwidth) than just having a copying collector rescue the live objects and then reuse those memory pages en masse.

Once your program's memory footprint is small enough to fit on the machines you're using, making it smaller takes more runtime work but has little benefit.