Actually, an acre was not meant to be square. The 10 acre square you mention is 10 chains or 1 furlong on each side. The Ideal Acre was defined as 1 chain by 10 chains (66x660 ft.) with the notion of that as a lot that you could build a house near the front and have a space to grow stuff at the back, all the while making access road at the front relatively short and putting neighbors close together. The 11 pops out of a chain being 66 ft long. I have never seen a definitive explanation that proves whether the system started from the top (mile) or bottom (chain). The rod (16.5 ft) is clearly a sub unit as 1/4th of a chain. If you have a sense of humor it is actually kind of fun- a mostly binary (quaternary in area) system with a sneaky 11 hiding in there. The system was created from practical usage, whereas the Metric system starts with a base unit and throws factors of ten at everything. For many things (cooking is a good example) a binary approach is actually more useful than the metric system.
Probably the biggest hang-up in the English measuring system is the dichotomy between the statute mile and the nautical mile which was only finalized after someone actually figured out how far it really was around the world. So now we have MPH and knots to deal with. The nautical mile is a practical measure in that it takes 21,600 of them to go around the world making 1 minute of latitude equal to 1 nautical mile. Are we having fun yet?
"not meant to be square"...no kidding!!! yes, you can make a clean "rectangular" acre (but at least it's not 6000x4000 anything).
Figuring this out (which I did a few years ago) made me at least see
an underlying logic to the system; it's kind of screwball in its own amusing way. It's not as "clean" as SI, but at least one can see where it came from.
I do recall reading that a mile was 5000 feet until it got bumped up to 5280 so that it'd be a whole number (8) of furlongs, so I suspect it was started from
both the top and bottom, when the inevitable collision occurred, "up from the bottom" won. (Wikipedia has a long explanation of this I don't have time to parse right now to check it against my recollection; but it appears the year was 1593.)
The nautical mile is now defined to be 1852 meters (note it's tied to metric); before it was 6080 (in the UK) or 6080.2 (in the US) feet, a bit over 1853 meters. So though the original intent was to be one minute of arc on the earth's surface, it isn't quite that any more. (Of course the meter was originally defined to be 1/10,000,000 the length of a meridian from the equator to the poles; it isn't any more either.)
Weight is another one with a lot of history. Apparently even groceries were measured in troy pounds of 5760 grains before Queen Elizabeth; after her reforms we went to the avoirdupois pound of 7000 grains (the grain itself not changing). However, the monetary system was tied to troy pounds (a pound sterling really was a troy pound of 0.925 fine silver at one point), so we couldn't stop doing precious metals in the troy system. And so we continue to have two different kinds of ounces to this day.
All well and good, until you start dealing with other countries with slightly different histories and therefore two different kinds of miles, pounds, etc. At that point it becomes an ungodly mess...so there is the rationale for the SI.