Why are Salesforce Visualforce pages served from a different domain?

If we see carefully, all our Visualforce pages are served like “c.YOURSERVER.visual.force.com/apex/YOURPAGENAME” ,
And because of this most of time we run into Same-Origin Policy error in Javascripyt if we try to access parent page from Iframe. Following reason is explained by one of the evangelist of Salesforce:

“The move to separate domains has one very specific purpose: leverage the browser security model (same domain policy) to protect our customers and the salesforce.com service from cross site scripting and cross site request forgery attacks.

Moving to the serving pages from separate domains is a critical component of our ongoing commitment to insure the highest level of security and availability for everyone.

In the world where everything is served from the same domain any custom page that you visit had full access to any other page in your org and also any page served from salesforce.com itself. This included potentially malicious code that was installed as part of a force.com package.”