Because usability. If you have the files downdown1down2downxyz and download and the user only knows that it was “something with down” it’s best to show the user everything matching “down*” and let the user decide what’s the correct one.
Also I’m not sure but wouldn’t your expression show everything if only one character would be entered?
And again I don’t see this solving anything if the entered string actually contains other characters then what’s in the file (D != d)
Yes one could argue that some form of advanced algorithm or even AI could be used to identify such use case like download and Download but this is programming Humor, not linguisic Humor.