The other page is necessary since those who have a Paypal account need to log in to Paypal to make their purchase - they don't need to enter their credit card detail at all. Not having to enter credit card details is one of the benefits of Paypal.
Also in most countries of the world Paypal require people to have a Paypal account to be able to buy through Paypal using their credit card - there are only one or two countries where you can make a Paypal purchase by credit card without first creating a Paypal account and only in those countries does the option of entering credit card details to make the purchase appear. All other countries require theperson to create a Paypal account first ant then link to their credit card.
So whether the person has a Paypal account or not and what country they are in determines whether the page you wish was first can be accessed at all.