It's easy and cheap to determine the original carrier (or its sucessor) for a US phone number. It costs money to do a porting lookup to determine the current carrier.

Most of the reason to deny voip users is that many voip services give phone numbers away like candy and then those phone numbers are used to abuse other services, so checking the original carrier tends to be enough for abuse screening.

Some use cases want more though. Banking KYC has some back channel to get subscriber identification or be alerted when ownership changes; those institutions may be willing to pay for current carrier lookups and deny usage of numbers where they don't have a back channel to the current carrier.