Why does the pro forma invoice not show VAT?

The pro-forma invoice is just a request for payment before the event or stay takes place, because VAT is not charged until the date of stay/event, you cannot show VAT on a pro-forma invoice. Instead, a VAT invoice should be sent out after their stay/event to show the VAT breakdown.

VAT cannot be shown in Hop until the item/article (Accommodation or Event etc.) has been charged.