Because McAffee pays to have their installer "bundled" with the update. It's not right, and the "blind installer" in the update (with no opt out) has been documented and reported.
You can download the Adobe Flash Player installer directly (without any bundled installers) by clicking one of the following links.
Bookmark those for future use.
It really isn't acceptable to not give the average user a way to opt out at install stage. The auto updater program I use does not take you to the links that Mike mentions so I then had to uninstall McAffee & am not happy.