As you have now worked out, QM2's transatlantic crossings are in themselves a 'destination cruise' - and people generally sail for the joy of being on board. As such, Cunard does not do transatlantic repositioning cruises. Other cruise lines need to move their ships between Europe and the USA at season cross-overs - and as such, the dates of such sailings are less frequent...
Hope you (i) find what you need (ii) discover the joy of a QM2 TA when budget permits!!!