We include it because the first 69 weeks took place in history as well, so also will the 70th. (It either happens during this part of history or in the millennium, there is no in between spoken of in Scripture, and it doesn't happen during the millennium as the 70th week is what ushers in the millennium.)
The end of the 70th week is when all is set right, and thus those 7 years are part of the time that Earth remains in travail and God is bringing the final judgments that will cleanse the earth and judge the ungodly. It is the end of the 70th week when Messiah's kingdom begins and thus begins the 7th millennium - the millennial kingdom and the thousand-year reign of Christ from Jerusalem.
I'm unaware of any scriptures that include the 70th week as part of the millennium, so it must come before that, and thus those seven years are part of the years counted until Christ returns.