This is incorrect. The original Japanese operational plan for a "Go South" initiative, the Nagano Plan, stressed, above all else, not going to war with the United States. Osami Nagano, Chief of the Naval General Staff, and the highest ranking officer of the IJN, was adamantly against a war with the United States (as was, for all intents and purposes, the entirety of the IJN). Thus, Nagano's original strategic operational plan proposed to divert around the Phillippines, and relied on the United States political process to forestall a war with the US. The assumption rested on the unwillingness of the American public to defend European imperialism (an accurate assumption) to delay or prevent FDR from declaring war, which in all likelihood would have happened. Even if Congress did agree to a declaration of war, the entire process is public and gives the Japanese time to prepare for it.
It was only after Admiral Yamamoto threatened Nagano with his resignation, and the resignation of the entirety of the Combined Fleet's senior staff, that Nagano was reluctantly shoehorned into Yamamoto's plan for Pearl Harbor. Ah, the irony. Almost no one in the IJN believed a war with the United States was winnable in the end, yet they still went ahead and pushed it through.
In any event, a Pacific War without a Japanese Declaration of War on the US puts Japan in a far better position. One of the greatest problems throughout the Pacific War for Japan was logistics. The Japanese merchant marine simply couldn't handle the effort of supplying a war in the Eastern Pacific. Whereas ships going southward to the DEI had the added benefit of transporting oil and raw materials and military materiel to and fro, those ships going to the Eastern Pacific came back home with nothing. Moreover, the inherent idiocy of sailing around the East Pacific looking for a decisive battle, burning up lots and lots of oil both transporting aforementioned oil and in actual patrols, when you're fighting a war because of an oil shortage, is rather stupid logistically. In this scenario, Japan isn't going to all these lengths to supply barren islands on the other side of the Pacific, and can fully devote towards utilizing and exploiting the riches of the DEIs. The United States will certainly declare war later then in OTL, and there wouldn't be as strong a political will to persecute a total war with Japan (no Pearl Harbor). In the event war with the US does follow, the Japanese would be in a far better position to combat the US, having had more time to consolidate their gains. Moreover, the USN is still greatly hampered, and can't operate far outside of Pearl Harbor (which was already stretching the US ability to supply the navy). Without a large enough tanker or transport fleet, the USN is effectively grounded for several months (since the battleships were never sunk, which magically lessened the logistics burden of the USN). The naval facilities in the Philippines were completely inadequate for maintaining the USN, as are the airbases, which means that any naval operation being fought in and around the Philippines would put the USN at a severe disadvantage (beyond the preexisting qualitative difference between the Combined Fleet and the Pacific Fleet). So, if political pressure forces the USN to act (which isn't unreasonable to assume), the Pacific Fleet would be operating in far worse conditions then they were in OTL. If they try to operate out of the Philippines, the USN and any aircraft faces an extremely long supply line (which would certainly be harassed by the gauntlet of Japanese held air bases and submarines), much as the IJN had to at Wake or Midway. In the event they operate out of Pearl Harbor, the IJN now has the benefit of operating out of Truk (which was already well-supplied and was equipped with excellent port facilities).
Certainly, if (when?) the US shifts into total war, they will begin producing transports like crazy and will certainly alleviate the distressful logistics situation, but this still buys the Japanese critical time, and helps their own logistic situation much better.
Not to mention it gives Germany more time before going to war with the US.
So there you go.