To avoid someone registering support@gmail.com, or any variation of it, for nefarious purpose. (Is this a real question or a joke ?)
It is a real question. I get that allowing support@gmail.com would not be a wise idea.
But it seems weird to forbid <randomstring>support<randomstring>@gmail.com.
That is a bit heavy handed but I am pretty sure people could be creative with "Gwalletsupport@gmail.com" or "googleplussupport@gmail.com"...