Why not attack British and American trade first?

Why would Japan not begin by attacking British and American trade?

Because reaching those trade routes required getting past the enemy fleets and their bases, so Japanese doctrine placed fleet defeat first.

Geography kept commerce raiding behind fleet defeat in Japanese planning. Reaching the main Atlantic, Indian Ocean, or Mediterranean trade routes first required passing strong British and American positions. Singapore and the Philippines therefore stood between the IJN and the trade it would want to destroy. Sea-lane access was expected to follow victory over the opposing fleets rather than create that victory.