Because, if the AI worked, it would consider the fact that if it changed its values, they would be less likely to be maximised, and would therefore choose not to change its values. If the AI wants the future to be X, changing itself so that it wants the future to be Y is a poor strategy for achieving its aims - the future will end up not-X if it does that. Yes, humans are different. We're not perfectly rational. We don't have full access to our own values to begin with, and if we did we might sometimes screw up badly enough that our values change. An FAI ought to be better at this stuff than we are.
I think assuming an AI cannot employ a survival strategy which NI such as ourselves are practically defined by seems extremely dangerous indeed. Perhaps even more importantly, it seems extremely unlikely that an AI which has FOOMed way past us in intelligence would be more limited than us in its ability to change its own values as part of its self modification.
The ultimate value, in terms of selection pressures, is survival. I don't see a mechanism by which something which can self modify will not ultimately wind up with values that are more conducive to its survival than the ones it started out with.
And I certainly would like to see why you assert this is true, are there reasons?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.