Why do I have to indicate right away whether I want an eSIM or a physical SIM card? Can't I just request both?

This is not possible. You can only request one type of SIM card at a time for a particular telephone number. If you are unsure whether your phone supports an eSIM, you can check here. If you have a new eSIM-enabled phone and you currently have a physical SIM card, you can request a replacement eSIM via your My Viking account.