Because Japan's oil pretty much came from the USA and the Dutch wern't going to give Japan anymore oil ((93% of her oil came from the USA). The only way forward was to seize the Dutch East Indies for oil as the alternative was too pull out of China as the US wanted and suffer a loss of face. But I don't think they got any from threatening the Dutch with force. One of the things tried very hard to do was avoid war with the U.S until it could no longer be avoided by refusing to pull out of China. And threatening the use of force completely counteracts that.
In any case, they knew that if they wanted the Dutch East Indies for Oil, neither the UK, but especially the US would remain on the sidelines and they would be fighting them even if they did not attack the US and UK and just the Dutch.
This PDF here goes into detail about it.
http://www.cc.gatech.edu/~tpilsch/INTA4803TP/Articles/Oil Logistics in the Pacific War=Donovan.pdf