Why navies build specialized support ships

Why did navies build purpose-designed submarine tenders and other support ships instead of converting existing hulls?

Because sufficiently specialized conversions could require so much reconstruction that a purpose-built auxiliary was cheaper and more effective.

Navies did convert merchant ships and old warships when suitable hulls were available. Specialist support equipment could, however, require extensive changes to the hull and internal arrangement. Beyond a certain amount of reconstruction, a conversion became more expensive than a new ship. Purpose-building was then the more economical way to obtain the required auxiliary.