G3 carriers cost more and complicate funnel trunking

Why not convert the barely laid-down G3 battlecruisers into aircraft carriers?

They were costlier near-new builds and their machinery arrangement made carrier conversion harder than the Admiral class.

Because little of a G3 had been built, converting one would amount to paying for a nearly new carrier rather than cheaply reusing an advanced hull. The idea becomes attractive only if a large American conversion program creates a treaty and parity need for more British decks than the Admirals can supply. The G3 machinery arrangement also places funnels where a carrier conversion would require extensive trunking or twin islands. Admiral-class hulls therefore combine more sunk construction with a layout that is easier to adapt.