"Where is the lake?"
They are teaching the word "lakes" and the difference between it and its singular form. :)
If there were no singulars, we would not know what plurals are. The lesson is about comparing them in order to teach the difference.
I think because "находится" means something more along the lines of "belongs there." A lake is somewhere whether or not it belongs there.
I can't see what's wrong with my answer. I said где оэеро and it says I have a typo.