Must they? Or are the organs and brain not particularly large and are just rearranged in the squishy container that makes up an octopus body.
I believe you are not a biologist, or expert on molluscs, and I’m relatively sure you didn’t take into consideration the actual size of octopus organs, or to what degree they can be rearranged, when you made this judgement.
That’s not to say octopus organs aren’t a lot tougher and stretchier than human organs, I just don’t think your assumptions about how much they are squeezed when the octopus changes shape are accurate.
What makes the human stomach so tough and stretchy? There are plenty of organs and biological systems that are tough and stretch, even in the human body. We’ve needed a stomach that is tough and stretchy, and have had no need for the brain being similarly malleable, so evolution has made it impossible to squeeze the latter through a small opening.
An octopus on the other hand has had completely different evolutionary pressures and therefore doesn’t have a huge firm human brain inside a hard skull. And possibly its much smaller central nervous system in its squishy body can change shape more easily, but if so that’s not all that surprising.