Legal requirement that they do in the UK (though in metric units), so it's £/L and £/Kg (or for smaller items per 100g and then some nuance on unit prices for things not sold by weight/volume).
It does make their shenanigans with multipacks and such way more obvious by design.
> The PMO also requires traders, where applicable, to display the ‘unit price’ – which is the price which would be charged for the relevant unit of the goods, depending upon how the goods are sold. Unit pricing can be a useful tool for consumers. It allows consumers to compare the price and value of similar products. This can be helpful when making a decision between similar products that are sold in different sizes or quantities.
https://www.gov.uk/government/publications/price-marking-ord...