They are both interim champs. If Usyk doesn't fight them which he won't they will get upgraded to full status at some point.
Not only that. Usyk is 38 years old and very much on the decline. Both Parker and Kabayel realize that the longer they wait, their chances will increase.
I don't blame them, really - even if a match-up between them would be great for us, the fans, it doesn't make the most sense for either of them in terms of a business decision for one to lose and be denied the big chance & payday rather than simply queuing up and both getting it by turn.
I understand it because they both want a shot at the title and a big pay day against Usyk. The reality is neither has a great chance against Usyk and a fight between the two of them would actually be quite interesting.
The danger of them waiting for Uysk who is unlikely to fight them for more lucrative trilogy $ is that they could be inactive for a long period or fight in an always dangerous can keep active fight at HW.