Why do many metal fans say that modern metal isn't metal?

I am a metal fan and have been for a good few years. I started with clic metal and NWOBHM (Iron Maiden, Black Sabbath, etc.), but as my music taste grew, I started liking bands such as, Bullet For My Valentine, Avenged Sevenfold, Slipknot, Korn, etc. I've heard countless times, that these bands are "not metal", and are just a disgrace to metal. I honestly don't understand why so many people think this. They have just evolved metal. Its still talented, pure, and packed full of emotion and reality. I mean at least they don't rely on computers and drum machines like many modern musicians. Just a thought :)