Why don't meters go into light years or parsecs nicely?

Anyways, an acre is the area people would plow by ox in one day. You measured fields by the acre because in a medieval society that said something really interesting about how many farm workers you needed for a given area.

Similarly, a mile comes from the Latin for 'thousand paces', which is a fairly natural way for people on foot to measure distances.

Much like how light year says something scientifically interesting about distances to stars so we use it instead of petameters or zettameters in astronomy, people used acres and miles despite them not going into feet well.

It works because you generally don't convert between light years and meters, or acres and feet. They mostly just exist at different scales.