It's boring but apparently when supermarkets were becoming a thing In the 60s an MP said the buildings looked like "nazi concentration camps" and they stopped getting planning permission.
When they were building a brand new town in Essex, they gave Tesco permission to build one in the town centre as long as they made it fit in with the local architecture so they based it on a local Essex barn and church bell tower.
Then they got lazy and just copy/pasted the design across the country.