I'm pretty sure there are alot of reasons why men flirt instead of ask a girl out... The same reasons a girl will probably flirt with a man and not ask them out....
I am pretty sure some big reasons are.. To shy, Afraid of rejection, and intimidation...
Although i'm pretty sure there are many more out there...
I personally do not think it's annoying.. I think flirting is harmless and sometimes it can be rather cute...