I understand that they didn't have the budget to make a bunch of massive creatures for the boys to fight, but is that the only reason? Maybe the budget? Maybe the fact it had to be a realistic fight for the boys? Anyone have any other ideas?
The plan of the Leviathans and in particular of their leader, was to infiltrate human society. This can only be achieved if they look human, so even if their original shape was a black goo or something, they took human forms.
Specifically, their leader took the persona of
Dick Roman an American businessman.