Why President-elect Trump has to wait for his inauguration

Donald Trump has won the US election, but his official inauguration won't be for 11 weeks. Why so long? What is the government doing between Election Day on November 5 and Inauguration Day on January 20?

Why President-elect Trump has to wait for his inauguration
Donald Trump has won the US election, but his official inauguration won't be for 11 weeks. Why so long? What is the government doing between Election Day on November 5 and Inauguration Day on January 20?