Why SQL Server 2008 R2 do full installation for just adding another instance on the same machine

Why SQL Server 2008 R2 does a full installation for just adding another instance (only database engine) on the same machine? I thought it will be quick a installation, but it's not. What is SQL Server doing, copying/registering all the files again for this new instance?

Please clarify me.

Actually Sql server has two types of component\features.

1. Instance specific features

1. Shared Features

And Database Engine, Analysis Services, and Reporting Services are part of instance specific features which has to copy many files for installing new instance.

http://msdn.microsoft.com/en-us/library/ms143786.aspx

StackExchange Database Administrators Q#81552, answer score: 7