Why does WWII seem to be two separate wars

Basically, because it was two separate wars, at least initially.

The war in East Asia was going on since the 1930s, with preliminary fighting happening as early as 1932, and then more full blown fighting by 1937. Leading up to this, Germany was actually present in the form of advisors to the Chinese forces that were fighting against Japan advances, but then later they were removed as relations between Germany and Japan shifted.

But it seems like the USA first was fighting in Europe - then it took on Japan.

The Second World War in Asia started out as the Second Sino-Korean war before WWII began properly, and this then was enveloped in the World War in Western sources. It's not that America joined up and then later decided to mess around with Japan. Rather it's that the two regional wars ended up combining in the view of certain involved parties.

America was a significant force at certain points throughout the fighting, but I'd be wary of making them too much of a central focus when dealing with the time period. They certainly were a much more minor player in continental Asia, and even just the bombing of Japan is a more complex issue for which you'd want to take into account other considerations that didn't directly involved Japan in the first place. I'm not qualified to speak to that in any great detail, but just be careful about oversimplifying WWII as an American endeavour.

Anyway, if you're interested in reading up on some of the involvement that certain German officers had and how the fighting played out in the Far East, I find Shanghai 1937: Stalingrad on the Yangtze by by Peter Harmsen to be a nice quick yet informative read. It also gets into the build up to the Battle for Shanghai which is being referred to in the title. He has a follow up that deals with the fighting in Nanjing, but I've not yet read it.

If you want more detail, check out The Wars for Asia 1911–1949 by S C M Paine, which gets into much more info on that whole time period, and has an introductory 10 pages entitled "The Asian Roots of World War II", being the main event the book is working toward. Chapter 7 is where it really gets into WWII as a global war.

It was decided that even before the US entry into the war to focus on Europe first. Germany was seen as the most dangerous fo the Axis nations. They were the most industrialized, and the only one with the ability to attack all 3 major Allies on their home soil at once.

At the Atlantic Conference in Canada in August 1941 FDR and Churchill came to this agreement. Then just after Pearl Harbor in January 1942 at the Arcadia Conference in Washington DC the top Brass formalized this.

Only enough forces to stem the tide or deal with emergencies would initially be sent to the Pacific, though in time the US's prodigious output and Japan's poor strategic choices, over extension, and inability to replace losses meant that momentum had shifted to the Allies long before VE Day.

It was, as you said, two separate wars. The US was not committed to war in Europe though, at least before Germany declared war on them first right after Pearl Harbor. Remember, economic sanctions on oil and material like steel, etc. were placed, directly prior to the war, on Japan and not Germany.

This has multiple reasons and results, but can be seen most obviously in the factories that the Ford Motor Company worked jointly with German auto makers and the German subsidiary of Coca-Cola Co. making profits even during the war with Fanta, a new product able to be made with the scant material they could scrounge behind the lines. American companies were not doing the same with Japanese.