I believe the biggest reason is to do with Oracle's past and range of supported platforms. The command line was the lowest common denominator. So with all of systems setup, configuration and administration being available as SQL statements has made it a very flexible solution.
Using GUI tools such as toad or enterprise manager, do make administration easier, but as you can see from the show SQL buttons you do get to see exactly what will be executed on the DBMS.
I do appreciate knowing a lot of these commands does take time and effort but they are worth it. As they have on occasion been the only way recover a database instance after parts of the environment have failed and prevent them from being run