MS added the UAC feature to make this happen as a default, so you are the admin but running with user rights While this may be best practice, it is not what users do. People should run as a user and then when needed, log out, log in as the administrator or otherwise invoke the admin rights.
Because of course, when running as administrator, the machine is wide open. This is not recommended because software can do harmful (virus/security) installs as well as you installing helpful stuff. MS, knowing that many users give their day to day user account Administrator privileges.
