The acre was used to subdivide up square miles. It makes more sense if you know 43,650 = 660 * 66. Also, 660 feet is exactly 1/8 of a mile. So once a square mile had been surveyed, you could split each side in half to get 4 squares of 160 acres. You could then split each of those again to get 40 acres (hence the "40 acres and a mule"), and then you could split them again to get 10-acre squares. Then you could split them into 5-acre rectangles, etc. The rectangles are good at keeping access to an existing road, although the skinniness isn't great. And all of these sub-divisions could live on the same grid.

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.