Not knowing why they did this, my guess is it's faster. When you go to site-that-requires-a-login.com Chrome can have the password box populated immediately instead of needing to ask Keychain for it.
Don't make people believe this bs, just have a look at how many people have problems with exactly THIS "feature": http://www.google.com/search?q=kwallet+chrome+opens including myself.
It reads and decrypts every password you have on startup.
Safari and Firefox don't try to get the password until there's a form to put it in, but that's not good enough for Chrome.
I only use Chrome for testing apps, never for real use.