Your memory is going to hold information that the computer might need later. The computer has no pressing need for free memory right this second, so it holds onto information it might need later in the hopes that it can avoid a subsequent disk access. This both avoids the effort of making memory free that won't get used anyway and reduces disk accesses, a double performance win.
For example, suppose you run a program and it completes. The program is still in memory. Your computer has two choices:
It can make the memory free. This requires effort to make the memory free, which harms performance. When the memory is needed, effort will be required to make it in use again, which harms performance. And if the program is run again, it will have to be loaded from disk, which also harms performance.
It can leave the memory in use. This takes no effort, it's already in use. If the memory is needed for something else, the system can directly transition it from one use to another, saving the effort of making it free and then making it no longer free. And if the program is run again soon, it won't have to be loaded from disks. Wins all around.
The more memory your computer is using, the faster it runs. That's why we add memory to systems to make them faster, so they can use more. Your computer is being smart.
And, by the way, this is real physical memory (RAM) we're talking about, not virtual memory. The statistics you pasted are physical memory statistics. You're not out of physical memory because your computer will make memory free if it needs to and, more typically, directly transitions memory from one use to another without ever making it free.