Defensive tool use in a coconut-carrying octopus (2009)
sciencedirect.com
sciencedirect.com
Ocean water corrodes everything, no humidity control for drying things, chemical control near impossible, no heat control (no fire), etc.
And given octopus use there intelligence to hide from predators, it may be difficult for tightly knit cooperative social groups to form.
A cluster of octopus will look like a great dinner for sharks and other predators
Unless we teach octopus to make weapons and war on sharks? Then octopus could eat richly & rule the deep?
Also, simple things like... Primitive humans used tree branches to build lances and things to fight predators. They also used them to dig and to build homes. There are no trees underwater. Octopuses have soft bodies and are not equipped for digging in the ways that humans were, especially without tree branches to help them.
Another obvious difference: one of the key technological discoveries that made humanity happen is the discovery of fire. It would be somewhat difficult to make fire underwater.
Maybe it really is just easier on land. That being said, many species have gone from sea to land or land to sea during their evolution. Not impossible that some octopus descendant could become a land species in the far future... But I have a feeling that evolving bones or a hard shell would he helpful there.
cue movie scene where an octopus, drawn by strange unheard noises, surfaces and witnesses nuclear bombs exploding just above the New York skyline from far away, and the dawn of a new era begins
https://ocean.si.edu/ocean-life/fish/bioluminescence
I'm wavering over the two perspectives here. It does make sense that a water environment places far too many barriers on all sorts of natural processes, chemical, & physical, that seem necessary for any sort of technology. On the other hand we do have another form of "terra-"centric bias here, this time regarding the 'alien' -- the essense of the question considered imo -- environment and its alien creatures. So maybe there are in fact possibility spaces and progressive trajectories for 'high technology and science' via entirely novel takes on environmental manipulation. So, sure, no fire under water, but they have electricity [and light sources] (!!) on tap. Maybe Octopus sages will hit on 'eletricity' before 'fire'. Maybe they will learn to farm and control large collections of eels to power stuff [or build logic gates ..]. Maybe they will develop PSI powers (a bit woo) but who knows.
p.s. turns out in fact they do use other creatures as 'tools':
My theory is Octopus became intelligent long ago, but that civilization kept killing itself off (ennui, nukes, whatever) and that’s why now they live alone and kill the selves after breeding. Only stable intelligent offshoot to survive.
In those few years, they arguably learn quicker than humans do.
You won't find a two year old child carrying around two items for future assembly and use.
You are possibly underestimating two year old humans.
Lego pieces, for example, are designed to cater to instinctual tool use in human kids.
But, these guys have a sort of ensemble distributed processing system which is pretty cool. (We do too. The feeling in the 'gut' is thanks to ganglia.) Specialized processors have less to learn and learn faster, maybe?
https://octonation.com/octopus-brain/
"Every time they eat, they risk severe brain damage if their meal isn’t well and truly chewed up! Luckily they accomplish this by slicing up their prey with their serrated radula plus breaking down their meal with enzymes in their saliva."
Evolution or God was watching out for us terrans. Looks like they have a built in fail-safe switch to keep them from ever growing bigger or getting bigger brains. /g