I agree with you. But this is different.
HikaShop allows you to specify the number of columns of listings with options in your menus and modules. Thus, the width is calculated dynamically based on how you configure the menu/module.
Normally, you should never have to touch that CSS.
Nevertheless, it doesn't mean that you can't override it. You can override with the !important CSS functionality like that:
.hikashop_product_list_item{ width: 50% !important; }
But in that case, I would recommend to rather change the listing to a two column listing in the options of your menu/module instead of trying to override it.