You're making the flawed assumption that these other investors and players in the market have performed sensibly and are not in a similarly difficult position. This is probably not the case. The question is not only what the financial risk this poses to Evergrande's investors/creditors, but what other companies are suffering similar negative pressures that have pushed Evergrande this far into the red to begin with. If this were an issue isolated purely to Evergrande, then yes, systemically, the loss would be relatively isolated and generally well absorbed. The issue however is more widespread and systemic though, and as a result, Evergrande isn't the only one in this situation.
I'm not a financial advisor, I can't detail with any educated accuracy what specifically has transpired, but generally, the CCP has changed it's position and policies on lending to real estate companies and speculation in the real estate market. This is a policy shift in response to negative patterns in the market, namely but not limited to: supposedly large chunks of the real estate properties in china are sitting empty as people are buying them as part of their investment portfolios, artificially inflating pricing and perpetuating development far beyond practical demand, leading to a potential speculative bubble. As a result real estate developers were taking on debt to build new projects to expand their portfolios of real estate that were largely sitting empty and being used to borrow against to build more projects, artificially inflating price in response to their own demand to borrow against it... You can see how that can become an issue when the entire industry starts perpetuating that trend. It's an issue not isolated to Evergrande, they're just the biggest, and thus highest profile.
As a result, the "contagion" depends how bad things get with Evergrande, because they're not the only one in this difficult position. The issue is more-or-less systemic, and Evergrande is just the bellweather of sorts by virtue of them being the biggest. Of course there's lots of nuance here, as each individual corporations exact situation differs from each other, as does how the party might choose to deal with them, a number of other factors in play as well. But this should give a glimpse into why this isn't just Evergrande sinking or swimming.