It's just about what you are used to. For people who've grown up with metric units, it's natural to work with them.
UK had been officially metric my whole life and EU were at the time mandating pricing in metric.
Supermarkets were very reticent to use metric - or at least flouted the regulation, presumably as prices seem lower in pounds (UK 'lb') than kilograms. Now I find they often don't show weights but hide behind pack sizes for fruit pricing; presumably to prevent customers comparing. They do still have scales but it's a PITA weighing half your produce.
Getting comfortable so I could 'feel' what a particular mass (and £/kg) of apples was, say, took effort for me. Same with heights, and shifting to using kg for person-masses too.
Not legal, btw, unless you mean e.g. 'Lemons £1 (12p/ea.)?
The usual complaint is that the denominators haven't been standardised, so you might get A selling lemons /ea. and B selling them /100g.
But then, when they are literally sold as a number (or a rough number) in a net, as opposed to being weighed and stickered, even if there were a /100g (or whatever) price, it'd be +/- 10% or something. (That actually is standardised, that funny 'e' symbol for net weight, but the tolerances vary with nominal figure, so I'm not sure off-hand what it is at 100g.)
In Australia supermarkets are obliged to have a pricing per 100gm for every packaged item on the shelves.
It's slightly smaller print, of course, but it's clearly legible and allows for a wonderfully simple price comparison.
EDIT: Btw, this story came out a while ago, and includes a fascinating montage clip from 1978 - a vox pop of UK citizens, obviously the most whacky people are chosen - but it's a magnificent blend of dumb (you'll use more petrol because you don't get as many kilometres to the gallon) and sad (I don't want to be part of a community).
https://www.theguardian.com/politics/shortcuts/2019/feb/05/w...
There are some exceptions, supposed to help market traders sell 6 apples etc. The supermarkets are using those, which I think was unexpected when these exceptions were allowed.