why do cache files take so much memory

Your memory usage look fine. The best read for memory usage is the graph at the bottom left "Memory Pressure". What does that look like? If it is green it is good and if red it means memory is depleted and it is using the hard drive for memory.

Use Activity Monitor on your Mac - Apple Support

"Cached Files: Memory that was recently used by apps and is now available for use by other apps. For example, if you've been using Mail and then quit Mail, the RAM that Mail was using becomes part of the memory used by cached files, which then becomes available to other apps. If you open Mail again before its cached-files memory is used (overwritten) by another app, Mail opens more quickly because that memory is quickly converted back to app memory without having to load its contents from your startup drive."

Hello gussie88,

Apple designed the system to take all available RAM. So, if you have RAM that isn't being used for anything, it will be used for cache until you run out of things to cache.

That's one way to look at it. Another way is that it is being recycled.