Remember how New York’s (physical) master keys became easily accessible[1] despite the fact that they were supposedly so carefully managed? All that effort, all that trouble, and now not only is there essentially no security at all but the master keys created a security hole that did not need to exist.
The security of encryption is similarly proportional to the security of keys. The fewer things you have to secure, the easier it is to keep them secret. The “master key” concept in New York only served to create something of great value that people wanted to acquire, and massively increased the risk when that fell into the wrong hands. Obviously the same thing could happen with an encryption key, except it is worse because you don’t even have to be in the same country as the source of the key to acquire it or use it.
[1] http://www.nydailynews.com/new-york/pols-public-outraged-sho...