Why would the US not declare war after being attacked?

Refer to this overview by /u/TheAlaskan, and another great post by /u/ParkSungJun. In their conquest for resources, Japan had viewed war with the United States as inevitable, and saw the Pearl Harbor attack as a preemptive strike.

As for your other question, it was Japan that actually declared war first, and they certainly would have expected a response in kind. However, the declaration itself never made it to the US embassy before the Pearl Harbor attack actually happened.