They were big as fark, and had relatively tiny arms that would've precluded things like writing. Look at a dolphin for a sec. They exist now, demonstrate quite a bit of intelligence, and couldn't lay a brick if their life depended on it. Their environment isn't conducive to things like complex tool use unless they go and do something so far completely mad and evolve another set of limbs, but this time have 'em turn into something useless for swimming.

Regardless of how smart a creature may be, it's pretty walled if it can't pass information onto future generations in a way that's more robust than a game of telephone. If you can't do anything with that intelligence beyond hunt or scavenge better, that might just be where you plateau because it's good enough. You just can't judge the potential of what's in front of you that way when it'd take a massive shift in body plan and/or habitat to ever maybe get a push in that direction.

I'd say other primates have a better chance than anything to reach a comparable level to us, but we're killing them all off too quickly for that to matter.