Timestamp display in macOS Finder
Explanation
That's because the file you created through the terminal using the touch command was named that way by you (not the system) upon creation. What happens is that upon creation, the system interprets your chosen filename and checks whether it contains any "illegal characters".

In macOS, the only "illegal" filename character is colon symbol (:). This is because macOS use these in path names e.g. Macintosh HD:Applications:Safari.app. Additionally, file and folder names may be up to 255 characters in length.

The replacement of : into / is an automatic function built-in to macOS to prevent file path corruption through inadvertently placing a colon in the middle of a file name - which would change the entire path. You will not be able to adjust this behavior.

What about Screenshots?
As for the screenshots, their file names do not contain any so-called "illegal;" characters and are hence valid.

Additionally, changing the screenshot file name is also very easy in macOS. In Terminal, run defaults write com.apple.screencapture name "Some Name" - replacing Some Name with your own text.