Because my prediction is that Trump will be sworn in, irrespective of the election results. If Trump does win the election, then we do not have to wait.
I really have to wonder why you would think that? Trump is a private citizen and nowhere in the line of succession. I'm as likely to take office in those circumstances as Trump is.
Present line of succession
Vice President Joe Biden
Speaker of the House Paul Ryan
President pro tempore of the Senate1 Patrick Leahy
Secretary of State John Kerry
Secretary of the Treasury Jacob Lew
Secretary of Defense Ashton B. Carter
Attorney General Loretta Lynch
Secretary of the Interior Sally Jewel
Secretary of Agriculture Tom Vilsack
Secretary of Commerce Penny Pritzker
Secretary of Labor Thomas E. Perez
Secretary of Health and Human Services Sylvia Mathews Burwell
Secretary of Housing and Urban Development Julián Castro
Secretary of Transportation Anthony Foxx
Secretary of Energy Ernest Moniz
Secretary of Education Arne Duncan
Secretary of Veterans Affairs Robert McDonald
Secretary of Homeland Security Jeh Johnson
This would of course change after the elections,, but there is ZERO chance Trump would be in it should he lose