But why don't I know?
No skin in the game. If I owned land then I suspect that I would know exactly how big a hectare or an acre was. If acres were the discussed measurement unit then those would be the units I would use for my understanding, much like how that in the fairly metric UK we still use feet and inches for comparing height.
In the UK people know their height in feet and inches. If you are a guy that is all to do with 6 ft. If you are 5ft something tall you are not 'tall', if you are over 6 foot tall then you are 'tall'. Nobody has the foggiest what that means in centimetres or metres.
There are also practicalities. I am okay measuring things in the garden with a stick. Then measuring that against paving slabs which are two foot or 60cm across. For me the paving slab is the unit. None of this metre nonsense.
Miles are also variable. Due to weather, traffic and foliage you can walk one mile between markers and it be different each time. But that doesn't make the measurement any less useful.
With all these legacy measurements they work great for casual use. Horses height? That will be hands, please. Building height? Storeys. Mountain height? Feet. 100 metres? Well that is 100 yards.
None of it is logical, we are not comparing apples and oranges with these vernacular units, the trick is to use them up to the point where proper SI units are needed.
See them as like the words we use for temperature. It is going to be hot/cold/warm/freezing today is fine, 'today could get to 23.2 degrees!' is just confusing even if well meaning.