Because China tried to push beyond the dotted line in several sectors, including Galwan, Hot Springs, Depsang, and even Demchok. In Galwan the conflict happened west of the dotted line. In Depsang,media reports indicated that India has been blocking Chinese patrols like China blocked Indian patrols. In Hot Springs the sat images are pretty self explanatory, and in Demchok the video posted by local nomads showed Chinese attempting to cross the river to reach the dotted line.
You are forgetting that there is still a lot of land China claims west of the dotted line, this standoff can be interpreted by the Indian side as a way for China to push the dotted line west. Remember that prior to September, one of the conditions China gave of restoring status quo ante was India vacating Dhan Singh Thapa. This is clearly because in the first map by Praveen Swami, China's 1959 claim goes through finger 2.
Yes China already seized most of the disputed territory in the western sector and following years later. Nobody is saying otherwise. Yet many analysts, including personal favorites here like Sawhney and Shukla, believe that China wants to secure beyond the dotted line. I actually agree with them, although they of course have a very different(and incorrect) opinion of where the dotted line is.