In Europe grocery stores are obligated to show the price/kg or price/standardized weight on the price tag.

It's better than nothing but not always enough. For example it doesn't help with products made up of different ingredients where some are more valuable than others and companies replace e.g. meat with more filler.

Also not all countries require the per unit price to be as large as the total price so you still need to make an active effort to accurately compare different items.