Nor when May took over from Cameron, when Major took over from Thatcher, when Callaghan took over from Wilson, when Home took over from MacMillan, when Churchill took over from Chamberlain or when Chamberlain took over from Baldwin.
It's extremely common in British politics for the prime minister to change without calling a general election. In fact there were only two PMs in the entire 20th century (Heath and Attlee) who both entered and left the premiership as the result of an election.
Anyone who complains about "unelected PMs" needs to read more history.