Why the United States still built battleships after Midway

Were American battleships built after Midway a waste of resources that should have gone to cruisers and carriers?

No: they provided certainty against Japanese capital ships, and their numbers were small beside the much larger American carrier program.

Post-Midway battleships did not simply represent resources stolen from carriers. Aircraft might destroy enemy battlewagons, but battleships offered a known heavy-combat capability in a war too large to base every formation on an uncertain assumption. The United States also built an enormous carrier force. Comparatively few additional battleships accompanied that programme.