As a foreigner, I have trouble understanding this.
How can the final amount of a transaction not be known when you are paying it? Is that one of the usual nonsensical shenanigans stores like to play with taxes in the USA?
Accordingly, this is what customers have become used to, and this assumption has been further baked into POS software.
Rather than forcing all POS vendors and customers to change this habit, the EMV standard was relaxed for the US (and a few other markets where this is common) to not necessarily include the final transaction amount in the payment data signed by the chip.
With chip cards restaurants now tend to bring the card machine to the table. This way you can enter the tip on the machine and complete the payment without the card leaving your hand. I don't know if this is because the machine is there anyways or because chip payments need to know the amount when the transaction is authorized.
In some countries, the switch from signature to PIN has coincided with the introduction of chip cards, and given that the terminal needs to be brought to the customer anyway for those (or vice versa), it's just simpler for everyone involved to include the tip amount right there.
This is different from having different amounts in the chip transaction and in the "actual" transaction amount fields.
The former is something that the card/phone can show and verify, as that is the actual amount you're authorising, even if you don't "spend" all of it.
You can also simply wait for the receipt to be closed before tapping, then the terminal will show you the full amount.
I - for example - use the pre-transaction tapping when buying groceries, it means I can just walk out when the last item is scanned.
It is a security risk - an attacker may add some expensive item to the session - but I think I can take it, the terminal will still show the total at transaction close.