The whole purpose of retracting the gear is because you no longer need it and it adds massive amounts of drag!
It was a relatively rare event during my flying career which only routinely occurred during recurrent training/testing. No matter how well a crew brief for a go-around the instruction from ATC will often take you completely by surprise and as with any unexpected manouevre the workload becomes quite high momentarily.
My money would be on PM not calling 'Positive rate', (standard Boeing call) and the PF, busy confirming in his mind the go-around profile, not noticing.
You would be amazed what you see in the simulator when recurrent or training manouevres are practised, the purpose of which is, of course, to give pilots sufficient practice to prevent errors in real life.