Why submarines did not become routine defended supply ships

Could navies build several submarine or warship supply carriers able to defend themselves?

They could make emergency deliveries, but Britain did not need a specialised force; fast minelaying cruisers sometimes handled exceptional runs instead.

Wartime submarine deliveries did not justify a dedicated supply combatant because Britain never reached a level of desperation requiring such a force. When exceptional deliveries were necessary, fast cruisers could sometimes carry the load, especially minelaying cruisers designed for high speed. The response distinguishes an emergency use from a repeatable fleet requirement worth building specialised ships to meet.