Japan's self-defense force is better-equipped, I will grant, but there are a few reasons.
One is that there is always a higher level China could escalate to. I mean, in the end game they have nukes and Japan doesn't. But even if it didn't get to that level, China is a totalitarian regime with a history of brutally suppressing dissent, and I also assume that China would be willing to lose a lot more lives to make gains.
Japan is a high-functioning democracy, and has been one of the most peaceful advanced economies of the world since losing the war. Before that, yeah, they were a definite bad actor on the world stage. But they haven't committed any military aggression toward any country in more than 65 years.
It is my personal opinion that there is almost no way that the people of Japan (where I live, though I am American) would support a war against China. Even if China just up and landed an occupying force on Japan's Senkaku islands.
In a democracy, that public support really matters. In a totalitarian regime, not so much.
I don't personally know that much about the military stuff -- how much would Japan's superior equipment matter in an actual conflict -- so I'd be interested to hear more knowledgeable people chime in. But in Japan, a war with China would be politically untenable unless China like, invaded Tokyo, or something. Which is highly unlikely.
As with the Philippines, I think the USA is the main deterrent to Chinese aggression. And these days it's a pretty shaky deterrent.