Why is it assumed on sectionals that the area outside of a 700 AGL Class E (Echo) transition area is Class E starting at 1,200 AGL?
Take a look at the legend on the side of a VFR chart. (You can get them for free at https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/vfr/ )

Class E exists at 1200' AGL unless otherwise designated as shown above.