How come there were so many military casualties from China in WW2?

I wouldn't say "ridiculous". There aren't reliable accurate numbers for the casualties of any of the major sides in the war in China, but to summarise:

Side	Low estimate	Middle estimate	High estimate
Nationalists	3 million	3.5 million	10 million
Communists	0.5 million	0.5 million	0.5 million
Japanese	2.5 million	2.5 million	3.5 million
Collaborationists	-	1 million	-

These are only military casualties, and can include non-battle deaths (e.g., disease).

If we exclude losses to the Chinese forces fighting on the Japanese side ("Collaborationists" in the table above"), the ratio Chinese to Japanese losses is, depending on the estimate, somewhere in the range from better than 2:1 up to 4:1. These loss ratios favour the Japanese, but are to be expected considering that the Japanese generally had (a) artillery superiority, (b) air superiority, (c) overwhelming superiority in armour, vs minimal anti-tank weaponry, (d) better supply, (e) better training.

Individual battles often varies from close to parity in losses through to the Chinese suffering 3 to 4 times the losses, supporting the general picture from the overall casualty numbers (e.g., Shanghai: 3-4:1, Taierzhuang: 1-2:1, Changsha (1944): 1.5:1, Ichi-Go: 4?:1).

so many videos about China losing hundreds of thousands of soldiers during battles, while the Japanese lost a significantly lower amount.

This sounds like Operation Ichi-Go, which was a disaster for the Nationalists. This was a very large offensive in 1944 (extending into 1945), which caught the Nationalists at their weakest. Their supply situation was at about the worst it ever was during the war, with supply from outside China depending on the air route from India, internal sources of supply at their lowest due to Japanese occupation of the most populous, productive, and industrialised parts of China, poor harvests in the last year(s), and the Nationalist economy was in tatters. Much of the fighting was in areas where millions were dying from famine (the soldiers did not have that much to eat, and where they took grain from starving farmers, the local population was unwilling to support them or help them in the battles to come). Nationalist military recruitment was at a low point (military pay was well below subsistence level due to inflation). The Nationalists' best-equipped and best-trained troops were in India and Burma. The Nationalist military command (i.e., to a large extent, Chiang) refused to send adequate reinforcements and supplies to commanders on the front lines facing imminent battle (or already fighting).

Ichi-Go is especially lacking in reliable figures for military casualties. More than in other battles, desertion was rife, and counting the soldiers before and after doesn't give a reliable figure for casualties. The 4:1 given above is a mid-range estimate; low estimates are about 3:1, and high estimates can be as high as 10:1, depending on the number chosen for Japanese casualties.

While Ichi-Go was a disaster for the Nationalists, it also wrecked the Japanese army. The Japanese stopped short of knocking the Nationalists out of the war because they reached the stage where they couldn't continue to attack. While the Nationalist losses were higher than Japanese losses, Japanese losses were damaging. Considering the disadvantages the Nationalists faced at the time, even a loss ratio of 4:1 doesn't seem ridiculous. The total Nationalist military casualty estimates varies from about 300,000 to 500,000. This is for a battle that stretched from the Yellow River to the south coast of China (over 1000km) with 8 months of heavy fighting (the battle started in April 1944, and officially finished in February 1945, although most of the fighting was over by the end of 1944), so considering the scale, the total casualties don't appear at all ridiculous.

Finally, there are later wars where one side faced an enemy with air superiority, armour superiority, and artillery superiority, and still engaged in widespread conventional warfare. Generally, such circumstances led to lop-sided losses. For example, in the Iran-Iraq in the 1980s, Iranian casualties were typically about double the Iraqi casualties. Closer in time to WW2, the Korean War also provides an example.