Redownloading mods

Can somebody please enlighten me as to why I can play FTA servers all night after downloading the mods then go to play the game again the next day and have to re download every single mod and then some of the mods fail when I download them again?

Happens on Old Guys too I would play the server after waiting hours for the mods to download, go to a different server with my mate then jump back on Iraq not even 15 minutes later and then I need to re download every mod and content pack all over again taking me even longer