The reason it sits at 8GB on idle is... optimization. The memory is there to be used, so the OS will use it to improve performance until it's needed for more important tasks.
That's a blatant simplification, and does not match reality as far as I've seen.
The OS only only has one large source of memory it uses "optimistically" - the file/buffer cache. But that's tracked separately, so it doesn't show up in normal memory usage numbers (unless you don't know how to read them).
The other source of "extra" memory usage is memory mapped executable files, which can be unmapped and then read back on demand. That's quite small though.
Everything else (mostly) is actual memory usage caused by actual drivers and programs (though it can be swapped, but that's a major perf hit).
The major reason for Windows memory bloat is the hundreds of inefficient services from both Microsoft and hardware vendors that run at startup. The "optimization" pool (let's not call it that way) is way smaller than that.
eg. pre-loading an application is a pessimization if there's not enough memory - not only does it permanently eat away a portion of the total memory due to the intricacies of Windows working set memory management, it will need to be swapped out when actual demand for memory arises, competing with other disk access.
The only actual "optimization" in Windows is Superfetch, and that barely works these days.
