Meaning of "Last Modified" date on files (installed from outside the AppStore)?
If you copy a file from one folder to another, that does not update the creation or modification dates. Nor does making a copy in the same folder. Try it!
The same is true for 'copying' a file from a remote location, such as a network drive or downloading from the internet.
The modification date is only changed if the contents of the file are modified.
It is arguably more useful to know that 'this is a copy of the file that was created on Tuesday, and modified on Wednesday', rather than 'this is a brand new file that was made on Thursday', just because you moved it or duplicated it.
This way, you know that the file's contents are the same as other copies in other locations.