Because people value walking up to the car and just opening it vs finding the fob in their pocket and pressing a button, and that convenience outweighs the small risk their car gets stolen which is replaced anyway via insurance.
https://www.cnn.com/2023/01/27/business/progressive-state-fa...
Down here in Australia it's been a legal requirement since 2001 that vehicles be sold fitted with immobilisers, and our versions of the Kia and Hyundai models mentioned in the article you link to are not vulnerable to this attack.
The engineering work is already done, it is pure greed on the part of the manufacturer to leave it out.