Why Japan could not simply bypass Pearl Harbor

Could Japan have ignored Pearl Harbor and seized Southeast Asia while fighting defensively around the Philippines?

No; the intact US Pacific Fleet could concentrate against the advance, which is why Japanese planning sought to neutralize it at Pearl Harbor.

The attack on Pearl Harbor formed part of the operational logic of Japan's wider southern offensive. An intact US Pacific Fleet would remain the largest naval force able to disrupt that advance. It could mass against Japanese operations in the Philippines and Southeast Asia instead of reacting from a damaged base. Japanese planning therefore sought to neutralize the fleet at the opening of the war before it could interfere with the seizures farther south.