If my disc got scratched, the publishers aren't going to send me a new disc.

So, why is it illegal for me to make a copy of my own disc so I can safely store the original?

Why can't I mod my Wii so I can store all my games that I legally purchased onto a HDD?

because 'backing up' your games is usually a euphamism for pirating

making backups is not illegal at all

do You also make backup of your books?

im pretty sure if you bought the game, and can prove it, it is not illegal to back up games.

ihad my diablo 2 disc crack in the player and looked it up (2000), bliz did offer to send new discs... at a fee 10$.

i have a 360 game that's unusable due to a scratch it got from the console...

I don't think its illegal. I think even pirating is not illegal if you have the original copy and a license.

But I am not sure. 

If it's not illegal, then why it is impossible for me to play from backup without going through modding my system? Which is not what I wanted to do.

Because Sony doesn't want to allow anybody to do it because a wide marjority who do aren't using a backup copy because a past disk was scratched but because they are pirating.

Its not illegal. The distribution of copies is illegal.