For things like compressors & EQ used in inserts what you are really hearing is the “wet” signal. At the effect’s output you are getting the modified signal and only the modified signal - it is not mixed in with the original, it is the effect and only the effect sound you are hearing.
Now there are some compressors that do have a wet/dry mix control on them. This allows you to do parallel compression. But if you were to turn that mix all the way to the wet position that would be functionally the same as if it had no mix control at all. It is the same as if you insert a reverb and turned its control 100% wet.