Because the morons who send me hate mail don't seem to get it, let me spell it out, in simple easy to understand language:
You can't get into the White House using an alias. A picture ID is needed to get through the door; the sort of access Gannon enjoyed requires a background pass. Therefore, if Gannon was using an alias, White House staff had to be involved in maintaining his cover.
Do you get it now?
Morons.