I request to please contact your website developer (or) third-party shopping cart provider to pass the variables "SOLUTIONTYPE=Sole" and "LANDINGPAGE=BILLING" in the SetExpressCheckout API call to make sure guest checkout is offered as often as possible.
Unfortunately, Guest payment checkout(i.e payment via credit/debit) is not applicable for every user. It's important to remember that guest checkout is not guaranteed for every transaction. PayPal runs a risk check to determine eligibility for guest checkout.There will be times when guest checkout is not available.
This is intended behavior of the product. Customer/buyer need to create PayPal account to process the payment.