Files certainly can be resharpened - it's an acid etch. One school of thought (currently fashionable in the USA) is that new files aren't sharp enough until they've been resharpened, so some people send their new ones off straight away.
Mostly though you just need to buy decent files, which means Swiss Grobert ones. US Nicholsons are OK, are are old-stock Sheffield files, if they're still fresh. Chinese and Brazilian are anything but.