Barium phosphate is written as Ba3(PO4)2. Why can’t it be written like Ba3PO42?

Any time you have a formula with more than one of a polyatomic ion, you put the formula of the polyatomic ion is parentheses. Since there are 2 PO4, you put PO4 in parentheses. Compare that to AlPO4 where no parentheses are used since there is only 1 PO4.