Here's why:

* Sulfur (S) typically forms ions with charges of +2, +4, or +6.

* Oxygen (O) typically forms ions with a charge of -2.

To balance the charges in a polyatomic ion, the sum of the positive charges must equal the sum of the negative charges. It's impossible to achieve this with SO5, as the total negative charge from 5 oxygen atoms (-10) would need to be balanced by a sulfur ion with a +10 charge, which is not a typical sulfur ion.

Common Polyatomic Ions of Sulfur and Oxygen:

* Sulfite (SO3^2-)

* Sulfate (SO4^2-)

* Thiosulfate (S2O3^2-)