Why wouldn't an AI modify its own goals?
The predictable consequence of an AI modifying its own goals is that the AI no longer takes actions expected to achieve its goals, and therefor does not achieve its goals. The AI would therefor evaluate that the action of modifying its own goals is not effective and it will not do it.
I find it highly likely that an AI would modify its own goals such that its goals were concurrent with the state of the world as determined by its information gathering abilities in at least some number of cases (or, as an aside, altering the information gathering processes so it only received data supporting a value situation). This would be tautological and wouldn't achieve anything in reality, but as far as the AI is concerned, altering goal values to be more like the world is far easier than altering the world to be more like goal values. If you want a...
A stub on a point that's come up recently.
If I owned a paperclip factory, and casually told my foreman to improve efficiency while I'm away, and he planned a takeover of the country, aiming to devote its entire economy to paperclip manufacturing (apart from the armament factories he needed to invade neighbouring countries and steal their iron mines)... then I'd conclude that my foreman was an idiot (or being wilfully idiotic). He obviously had no idea what I meant. And if he misunderstood me so egregiously, he's certainly not a threat: he's unlikely to reason his way out of a paper bag, let alone to any position of power.
If I owned a paperclip factory, and casually programmed my superintelligent AI to improve efficiency while I'm away, and it planned a takeover of the country... then I can't conclude that the AI is an idiot. It is following its programming. Unlike a human that behaved the same way, it probably knows exactly what I meant to program in. It just doesn't care: it follows its programming, not its knowledge about what its programming is "meant" to be (unless we've successfully programmed in "do what I mean", which is basically the whole of the challenge). We can't therefore conclude that it's incompetent, unable to understand human reasoning, or likely to fail.
We can't reason by analogy with humans. When AIs behave like idiot savants with respect to their motivations, we can't deduce that they're idiots.