Why are the 'things' that are created from human transmutation so randomly generated?
Essentially, human transmutation was depicted to be impossible to do.

The reason why it doesn't look what they intended it to look like is because they just screwed up that hard.  

What they ended up with was something kinda sorta vaguely human, but only barely.

From looking at the picture you've posted, a lot of the pieces of a human are there, but they're not all correctly done and they're not all in the right place. It really doesn't look that random to me; if anything, it looks like a predictable (while still major) screw-up.