If a Mac has 1GB inactive, and it has been idle for an hour, why it doesn't swap the inactive RAM to the pagefile for "readiness"?
The system does not know which action will be the next. If instead of opening a new application after an hour you decide to work on some stuff that was cached (and is part of the inactive memory) it will be already there.

Instead of swapping out to free memory or emptying disk caches the system waits to see if you actually need the memory to avoid unnecessary work.

You can always force the disk caches to be flushed (freeing some memory) with the purge command.