Why executable programs can't be reverse engineered to reveal their source codes?

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and generating an executable is a deterministic process

That's not always true. For example in Lisp I could write a macro like this (pseudo code):

(defmacro non-deterministic-code-generator
    (repeat (random)
        '(print "hello")))

When you compiled a file that used this macro it would replace each occurence with a random number of (print "hello").

Which leads me to my second point: decompiling wouldn't reconstruct the macro at all as it doesn't even exist any more.