Why do frames kill SEO?

I've often wondered just what is it about frames that throws the SE bots so. I assumed it was because once a bot followed a link in the nav document, the content document often doesn't contain links back to the main site, and so was a dead end for the bot. But suppose one DID include links, even invisible to the human eye, on the content pages? Would that alleviate the problem bots have with frames?

I've wondered that too. It's interesting since PHP coding seems have only a little more information on it than a Frames page. That code usually grabs information from somewhere else too, meaning, atl east in my mind, the bot doesnt see any real page info, just script fetch data elsewhere.

And I am another one with this query.

I've have created them before on sites, and it was then I realised it's a completely different web page to the other(s) on the 'same' page.

My guess was that as they are completely separate, different URL and everything, I figured that it pretended the 'whole picture' from being seen by the bots, just 2 different pages completely unrelated in their eyes.

I would be interested to hear the reasons from a professional.