Wow, is this thread really necessary? It's been explained already by Hargreave. They EVOLVED in the ocean.
They adapted to the human conditions in order to be resourceful and proper. Remember, the aliens are ANCIENT. So for all we know, they may have been roaming around in the Ice Age and then have went to sleep inside Earth....they wake up, and Earth is no longer an Ice Age

So they must adapt because CLEARLY they need a lot of resources (remember, they were energy hungry in the first game...that's why the nuke actually HELPED THEM A LOT).
But anyways yeah....THEY EVOLVED and became human-like forms to be able to effectively battle the humans in larger numbers with more fluidity, diversity, and etc., rather than floating in the air shooting random freezrays out. It took them a long time and a lot of resources to freeze the island in the first game....they can't freeze all of Earth and destroy the humans >_> Also, that big group of aliens in the end of the first game started traveling the world and they were ALL defeated by humans (it's explained in the Crysis 1 multiplayer map descriptions.....interesting place to explain what happens after Crysis 1's story but yeah.).
That post just reminded me -
What the hell happened to the aliens' immense need for energy?
And wasn't the alien structure a giant energy sink / beacon? In fact, wasn't the structure supposed to be signalling a larger force from the aliens' homeworld?
And to counter your post, it didn't take a long time to freeze the island. Did you even play the game? It happens in a matter of minutes, only to expand further following the nuke.
The aliens in Crysis were interesting and original. The "Ceph" of Crysis 2 are meant to fit a stereotype more than anything else. It would've been f u c k a w e s o m e to see New York frozen rather than infected by some goop.
To "Pull A Crytek" is to retcon an interesting plot, to dumb down gameplay, or to avoid innovation at all costs.