In a comment, John Lawler wrote:
It’s a fixed phrase. They are not subject to the usual rules of
grammar because they constitute a fixed meaning in frozen
form. It’s an attempt to emulate Chinese pidgin (don’t ask
me why), and you already know what it means. Just don’t assume
it’s normal English.
It’s suitable for an intimate relationship, in which
friends understand each other no matter how they speak. It
already assumes the speaker has known the addressee for a long
time and is glad to be reunited; leaving out words is a mark of
familiarity and intimacy. It is definitely not formal,
though it could be used between old friends in any context.