Migration in rails

Let us say I have a initial migration which is created during the model generation

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name
      t.string :email

      t.timestamps null: false
    end
  end
end

and sometimes later I want to add a field password in this migration like

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name
      t.string :email
      t.string :password_digest

      t.timestamps null: false
    end
  end
end

without creating a migration file rails generate migration add_password_digest_to_users password_digest: string can I add t.string :password_digest field to the initial file and run rake db:migrate command.

Why do we need to create a migration file to add this, instead of just adding t.string :password_digest and run bundle exec rake:db migrate

Isn't it overwhelming to create a 20 migrations, when we required to add 20 fields to the initial migration for a single model.