The checkout wizard offers three choices for the type of transaction with authorize.net, 2 local methods (AIM, and Secure Source Credit Card) and one Remote Method (SIM).
The difference between the Local and remote checkout method is where the user enters the credit card information.
In a local checkout scenario, the user enters credit card info on a page on your site.
In a remote scenario, the user goes to the gateway site to enter credit card info.
Of your checkout page does not have fields for credit card data, it is because you selected the remote checkout method for authorize.net (SIM).
the same would happen if you selected to use PayPal Payments Standard instead of PayPal Payments Pro.