Because these exps are paid/payable to persons. person includes Sole Proprietor, Partnership Firms, HUF, Co; etc.Personal accounts always appear in B/S until it is transfered to expenditure by debiting to P&L A/C.
For material/Goods Purchases, the Po is required. In the Po, QTY, Product Name, Where is delivery price, Terms and Conditions. In the po there is no matter of final amount. It is just the approximate amount.