Oracle Database Discussions

Hi,
At first i want to say that i'm new in oracle environment,especially in database administration field.

I want to disscuss about shared server

Oracle says that shared server gives benefit in memory,however by allocating for sessions UGA in SGA it avoids from using more memory,but in dedicated server ,where for every session Oracle creates PGA in memory.
My question is that:
While Oracle is able to share sessions data in SGA,as it does in shared server ( by allocating for sessions UGA in SGA) ,why it doesn't do it in dedicated server ,by that way to run away from consuming memory ?

In real it is possible only for memory problems we need to configure shared server in our db.

Sorry for grammatic mistakes :(
Thank you