Windows assumes that physical memory has no cost associated with using it. It makes no effort whatsoever to make physical memory free because this makes no sense. If it makes physical memory free, only one of two things can happen:
That memory isn't used any time soon. In this case, the effort of making it free was wasted.
That memory is used soon. In this case, the effort of making it free was harmful, as now the system simply has to undo that work to use the memory.
Modern operating systems only make physical memory free if they have absolutely no other possible choice.
If you're thinking "I need memory free now so I can use it later", forget it. You can use memory now and use it later. There's no painful tradeoff that you have to make here.
Everything is fine.
If you want a more detailed understanding of how physical memory is being used on your system, try RAMMap.