why EOS designed to register the key pair seperately to the main net?

[briantrudeau43]

when a user creates a key pair, it is natural for the key pair to be registered automatically to the main net.
but EOS requires 2 steps : create the key -> manually register the key to the main net.

why 2 steps??

moreover, registering the key to the main net is very hard for an normal user.
so, most of users should use a "registration" service provider with the fee.

why the EOS team (Blockone) does design like this??

i am not talking about the benefit from the registration of the key as an EOS coin owner.
i am talking about the reason behind this platform design.