I have always found the PayPal payment page to be confusing and biased towards getting people to sign up for PayPal.
Add to the mix that one way to pay for your PayPal service is using your credit card (but you first have to sign up for a PayPal service to do so) and it adds to the confusion.
Add to the mix that PayPal now offers PayPal Credit cards (https://www.paypal.com/us/webapps/mpp/credit-line-and-card-services) and you can expect even more confusion.
I expect that your clients who feel they must sign up for PayPal to pay are getting confused by the badly organised and marketing biased payment page.
My suggestion would be to try it out yourself and take a screenshot of the payment page, circle what the customer needs to press to pay by credit card, and add the screenshot to your website checkout page.
Alternate possibility is if PayPal no longer supports credit card payments in specific countries where fraud is high. Check to see if the people who felt they had to become a PayPal member to pay were from countries with less PayPal coverage.