Why navies preferred purpose-built carriers by the Second World War

Would an older capital ship such as HMS Hood have been converted into an aircraft carrier?

Probably not; by the Second World War the Royal Navy preferred purpose-built carriers, while its earlier conversions largely reflected available hulls and treaty constraints.

By the Second World War, major navies generally preferred purpose-built aircraft carriers because a clean-sheet ship used space and material more efficiently. Japan remained unusually willing to pursue conversions, while Italy and Germany also explored them to a lesser extent. Royal Navy ships such as Argus, Furious, Courageous, Glorious, Eagle, and Vindictive reflected inherited hulls and treaty or construction constraints rather than an ideal design path. The comparison separates the experimental and treaty-driven carrier era from later procurement practice.