Hello!
We teach it this way (in the sense of the original poster, i.e. gear down until a landing straight ahead is no longer possible) simply beacuse most of "our" examiners want to see it that way. Who are we to question the wisdom of our authority? At the low speed of the typical initial climb out of a retractable single or light twin, the gear does not generate much drag anyway.
The whole topic is purely academic, because after their training is finished, most of our students will never again fly an aeroplane that leaves them with the option of landing back on the departure runway once airborne (if they are lucky enough to get a job at all).
Happy landings
max