Like if there was no surface on earth, and only fish, there must be some very significant reason for advanced fish to even want to leave the water, let alone the atmosphere
Building a rocket requires hands, and the type of intelligence that evolves only after having hands.
The ocean floor has plenty of stuff to dig into, pick up, and manipulate, along with un-anchored things like mats of seaweed.
> Land animals are more likely to develop hands.
I can easily imagine sea-creatures making the same kinds of assumptions in reverse: "Sir Blub-blub, while this hypothesis of 'land' animals is indeed intriguing, they would undoubtedly be primitive, far less likely to develop intelligent grabbers. After all, there will be nothing worth grabbing but hard 'dry' rocks! They wouldn't even be useful for propulsion, given the intangibility of this 'air'."
Is there any particular reason why an intelligent organism couldn't evolve to be able to grow and change its body into any arbitrary size and shape that it wanted to merely by thinking about it?
Perhaps aliens from another planet would consider our limitation as four limbed bipedal organisms to be absurd.
Why can't organisms chose to grow eight hands each with 16 opposable digits?
It's pretty normal for organisms to have drastically different body configurations through their lives. e.g moths
Though I'm not aware of any that have choices to make in the process.
Edit: actually lots of organisms can "choose" to change their sex