Because many of the "superior" offerings are very expensive. Not difficult to understand.
You don't get much for the extra money as a woodworker but I guess an engineer might find them more useful.
PS I was pleased to read Sideways' comment about 'balance' I thought it was just me being a bit obsessive.
'Balance' gets mentioned a lot with hand tools but usually without much meaning.
or, you can just be like my English friend here was. Buy something mid market, have no way to check it, have struggles with it trying to do fine work (not everyone is hacking around builder's pine) using it and using it to set up jointer fences, and then later buy something good.
I don't see any cheap hardened head squares on ebay UK or I'd have posted them here, but there is a gorgeous browne and sharpe set for about 75 pounds.
that's past my limit of taste, but it isn't a tool that will lose value.
it doesn't need to be an expensive square, either, just something verifiable (drafting triangle if they're that straight would be just fine. I have trouble believing draftsmen would tolerate a plastic triangle off a hundredth of an inch over a foot).
The US appears to be an easier place to find vintage measuring stuff. My starrett 24 inch try square is super magic for checking the squareness of big stuff (framing squares, jigs, etc - can't say I've made too many jigs, though - one every five years) - and it cost $1.05 an inch or a little less ($25 - it's probably close to a dollar a pound).
hardened head combination squares here are $50 - the 6" squares that I have that look like yours, or bigger, are not so square. I don't know if they were dropped or the wood moved or what, but I wouldn't do clean work with them (they're site tools) without correcting them and wouldn't dream of using them for toolmaking.