No Log-in standard for Social Networks

As someone who logs in almost twice a day into his social networks, I must say that it gets frustrating that there is no standard login procedure for the most popular social networks.

Twitter:

1. Does not let you log in using your email address, only your username.

Yahoo:

1. Has an extremely complicated password standard, hence you will need to make a specific password for Yahoo which can't be used anywhere else.

 Facebook:

1. Only wants email address as the login, not the user name.

 LinkedIn:

1. Only wants email addresses instead of username.

 Why can't they allow both?