Crew, like all of us, do sometimes become a little complacent. as Littlegirl has stated, once this announcement has been made the crew should be sat down and not "finishing up". The 3 dings is a final reminder but, pilots, like all of us occasionally forget or miss things. Crew sometimes think they know how long it takes to taxy out, but these things can be hugely variable, I have taxied for well over an hour at JFK, but I have also been airborne from JFK 7 minutes after starting to taxy - now that was a rush for the crew!