I'm not sure where you read this, but it definitely wasn't easy for the Japanese. The Battle of Shanghai took 3 months to complete, and it took the Japanese another year to capture Wuhan (in a battle that took 4 months to complete). The Wuhan battle alone cost Japan more than a hundred thousand men, a third of the force participating in the battle. This, combined with multiple defeats in Taierzhuang, Suixian–Zaoyang, Changsha, Guanxi and other areas, ensured that the Japanese didn't launch another major offensive until 1944.

That means almost 6 years of stalemate, in which Japan controlled a lot of coastal cities but virtually nothing else, with almost no consistent presence in the Chinese countryside. The Ichi-Go offensive of 1944 was pretty successful for the Japanese, but it ended up exhausting their remaining forces and by 1945 China was steadily regaining ground until the Japanese surrender.

The original Japanese goal of conquering all of China was already pretty hopeless by 1940/41. The Japanese simply did not have enough manpower or supplies to control more than they already did, and while the Chinese did not have enough materiel to launch an effective counteroffensive they could afford to wait and grind down the Japanese forces via guerrila warfare.

So no, "easily" does not apply here. The Chinese stopped Japanese offensives for almost 6 years, I would say that counts for a lot.