why a!fileuploadfield can upload file at Interface but can't on site?

Yes, it has to be on either a start form or user input task in the context of a process model. 

In particular, this is because any file you upload to the upload box is kept in a hidden temporary location in the Appian filesystem and not moved to its target folder until the form is submitted, again in the context of a process model.  Thus even if you were able to enable one on a Site, it would never actually work, in terms of uploading your file where you want it to go.  When you're looking at an interface in the Interface Designer, however, the functionality is slightly less locked-down just so it's more testable for designers.