cause how useless would it be to return to the exact spot it was thrown from if, in the midst of fighting, Thor moves or is tackled or is forcibly moved elsewhere on the battlefield and he isn't there any more? It's not a boomerang !!!
Mentally, Thor controls where Mjolnir returns to him, cause we've seen Mjolnir change course during return fligh back to Thor and make course corrections due to Thor's changing placement on the field.
The "spot" from which Mjolnir is thrown is Thor's Hand. Not the Space in which Thor was occupying.