Because that's the legal side of this. You can compile and run GPL software on your computer without agreeing to the licensing terms, but you may not do it. It is very unlikely that someone will check your HDs for offending uses, and besides, any use of GPLed software assumes an implicit acceptance of its license, so if such a check is happening, you would have to be there to say: "I have it but I am not agreeing to the terms". That sounds a bit silly doesn't it?

That's how the rules have been made, and not complying could be assimilated to piracy.