why do electorates divide nearly 50:50?

Have you ever wondered why so many recent political votes divide their electorate almost 50:50? This is my attempt at a simple—perhaps obvious—explanation. The results of the referendum for Scottish Independence (No 55.3%: Yes 44.7%) and the Brexit Referendum in the UK (Leave 51.89%: Remain 48.11%) seem to me remarkably close—especially given that in one case you had 3.6 million people voting, and in the other 33.6 million. Why were these not 60:40, or even …