Here's why:

* Sulfur (S) has a maximum valency of 6, meaning it can form a maximum of 6 bonds.

* Fluorine (F) has a valency of 1, meaning it forms only one bond.

For a compound to exist, the valencies of the atoms must be satisfied. In SF7, sulfur would need to form 7 bonds, which is impossible.

The closest existing compound to SF7 would be SF6, Sulfur Hexafluoride.