Why can,'t I purchase more than one song at a time?

You can click or tap(depending on device) on each song you want to purchase. It will be charged to your payment method or account balance. Apple will then usually group purchases made on the same day into one email receipt.

Otherwise, you can simply purchase an entire album in one go by tapping on the album price.

There is no option to select more than one song from different albums at a time for purchase however.

Why can't you? What happens when you try?

How are you trying to purchase multiple songs at a time?

You can either buy individual songs or a complete album if you want and if its available for purchase.